One of the deplorable things that's happening on eBay these days is that there are scads of people who download manuals in PDF form from sites where they're free, then turn around and sell them for outrageous prices on eBay.
There are some naive people out there who actually buy these, not realizing they can get them for free if they look around. If you're pressed for time, or you want a paper copy, and the price is reasonable (under $5) then it's not such a bad thing. I've seen people trying to sell copies of manuals for over $100 and you can bet they didn't pay anything for the original.
To be honest, I don't know the circumstances. Maybe these are single mothers trying to support families selling manuals, but I doubt it. Some have made considerable investments in buying covers to make their reprints look like originals.
The only way I see to put a dent in this is if someone with time and bandwidth were to collect manuals from wherever, pack them onto CDs (or DVDs) full of manuals then sell them at cost, like for $5 or less. You wouldn't make any money, but it would be good Karma. I'm on dialup, don't look at me. Make selling manuals not such a lucrative business. Information should be free.
Try Googling for something like "DX-150A" "service manual" using the quotes to tie words together. Even then you'll probably find places where there's mention of a DX-150A and a service manual for something else. Work your way through the first 3 pages or more of hits before you give up. I've found treasures on the 6 or 7th page. I don't know what they sort by, but popularity is probably a factor, and probably not anything you care about.
If that doesn't work, look for a Yahoo or Google group that deals with whatever you're looking for. Join it and look through the files area carefully. Because space is limited some groups have spin-off groups that exist only to house manuals in their files sections (they're usually mentioned on the main group's front page). At this point I belong to over 40 Yahoo groups because I was looking for information on something. Only after you've exhausted other possibilities post a question with a subject like "looking for manual for xxxx". Some of these groups have thousands of members, and some get emailed everything that's posted, so try not to ask dumb questions. Do a little homework first. You might get emailed a manual, but more likely someone will post a link. If you own the equipment, being in the group will come in handy for asking questions about it.
There are people who are public-minded enough to maintain sites where you can download manuals for free. If you look at the declining cost of hard drive space and increasing availabilities for static IP addresses, it doesn't take much math to figure out that anybody can take an old computer, stick a refurbished hard drive in it, load a free operating system, and have a website. With terabyte drives becoming common space (per megabyte) has never been cheaper. Don't trust a drive that's been run for more than about 3 years, and consider a backup strategy or at least a RAID.
If the equipment is recent, don't overlook the manufacturer's website. Tektronix, Icom and Radio Shack for instance have manuals online for recent stuff. (Radio Shack has owner's manuals, they mostly restrict service manuals to their stores.)
This is just an observation, but I've noticed that there are PDFs you can search and some you can't. The ones you can search were mostly generated from programs like Word feeding something like PdfCreator and the others are images turned into a PDF like if you scanned it yourself. In cases where the original text is available to the PDF creating process, the PDFs are smaller and searchable. With DJVU files they tend to get OCRed when they're created so they're both small and searchable. Manufacturer's sites would be more likely to have good PDFs, but they don't keep them around long. A PDF is just a container and I've never seen one smaller or better than the files that were put into it. I just got a copy of a 318 page Tektronix manual that's only 6,013,442 bytes. The text is plain text with some formatting applied, the schematics look like about 100 DPI but they're still legible. The manufacturer is in a better position to make such things than anybody else because they have access to the original text in electronic form (unless it's too old for that).
Here's a list of some manual sites I've run across. Everything is free for the download, although some sites require registration first. Many specialize somewhat. You won't find Tektronix manuals on mods.dk for instance. Most have easy ways to upload and are grateful for uploads.
I was thinking of putting a list online, then I ran across AA4DF's list, so much of what's below is also there. Some I have experience with, some not. AA4DF unfortunately is now a silent key. I found his site, then 2 days later couldn't get to it and the domain was for sale. It was in a post by his daughter on qth.com's forum dated 4/9/2011 that I learned of his death in October 2010.
This list doesn't look very organized, and it isn't, partly because I'm not sure how to organize it. These links checked by me about 10/27/2011.
If what you're looking for is old, maybe old enough to have tubes in it, start with BAMA (Boat Anchor Manual Archive) which has been around for years and has quite a collection. Rumor has it they're not interested in adding solid state stuff, although there's some there.
A relatively new site is Ebaman which requires registration for at least some things. The only manual I've ever scanned and uploaded went there.
A good site for Tektronix manuals and some ham gear is KO4BB I've gotten some wonderful high-res Tektronix manuals there, and this wallpaper (with permission). Some are AA4DF scans. Supports http REGET, I've downloaded big files (over 80 megs) over several days on dialup with wget.
For Hammarlund stuff, see http://www.hammarlund.info
Fox Tango International Yaesu manuals, including the FRG-7
mods.dk has a fair bit of radio stuff, and of course, mods. They use a quota of things you can download per day, and if you get disconnected in the middle of a download that still counts. If you can upload something they don't have, that helps.
eServiceInfo manuals for consumer electronics and test equipment
Another site, which supposedly requires an invitation to register is ElektroTanya. The site's mainly in Hungarian, but supposedly you can change the important stuff to English if you're registered. You can do a search without being registered. I've never gotten in.
http://www.antiqueradios.com/archive.shtml Has what you'd expect from antiqueradios.com. I keep getting sidetracked: I got service data for 1946-56 GE radios there the other day, a 275 page DJVU file that's 4.6 megs.
http://www.hparchive.com A non-HP site with archives of old HP manuals
Tektronix Icom Fluke Radio Shack Company sites, current stuff
AC6V Big list of URLs to manual sites, too many for me to check on dialup, but some don't work anymore. That's the way it goes with old URLs.
http://www.bitsavers.org/pdf/ Has some manuals for mostly old computer stuff.
http://www.zen22142.zen.co.uk/schematics.htm From AA4DF's list, they have schematics (294 as of 10/27/2011) for things they design but I didn't see any manuals.
Not a manuals site, but has old books from tube days that the copyrights have expired on: http://www.tubebooks.org/technical_books_online.htm Doesn't seem to be particularly fast1.
http://www.tuberadio.it/docs.htm Old radio books, seem to be mostly from the 30s.
http://www.otari.com/download/index.html Product datasheets and article excerpts (10/27/2011)
Another general information site is http://www.tpub.com with "thousands of PDFs"
A PDF search engine: http://pdfflare.com/
Briggs & Stratton Kohler engines Tecumseh
A camera manual site.
wget, my favorite downloading tool source code or Windows version (non-GUI)
Another ham with a manuals site discovered 5/2/2013: KB7KBT. Has some owners manuals, service manuals, programming manuals. Has an IC-7000 service manual: I had to get mine from a Russian site years ago.
http://www.eserviceinfo.com/ used it 8/10/2016 to get a service manual for a Marantz SR5000 AV/surround receiver I got at the dump about 5 years before. I was about ready to scrap it for parts because I couldn't find a manual when I got it. They have forums on electronics repair also, nice site. I donated $5 once I'd downloaded my 16 meg manual PDF. Looks like a lot of recent stuff.
So spread the word. If you've got a website put some of these links up somewhere.
I'm on dialup, so everything is slow here. But I've got a free Unix shell account on devio.us which I use for downloading big things. I log into my devio account, use wget to fetch what I need, do an md5 checksum of it, then split it into 100k chunks, md5 those, and list it all into a small text file. Then I download the text file, add paths to make the filenames into URLs, and feed it to wget on my laptop. If I get disconnected in the middle or need to spread the download over 2 or 3 days, wget keeps on getting the chunks at about 35 seconds each. When I get done, I do md5 -c on the md5 of the parts to check them, then cat parts* > whole to recombine, do an md5 -c to check the whole thing, probably rename it, and I'm finished. I don't know where devio is or how they're connected, but I never noticed them being slow before. I got a 13 meg file from tubebooks.org and it took 35 minutes just to get it onto devio. That's faster than dialup which would be about 1 1/2 hours, maybe heavily loaded DSL or cablemodem.
These are from the National Weather Service that we have here in the US, about the only thing I bother with because they're very accurate and there's no advertising. These are links for my area, you can click on one, move it to your area, then bookmark that. The radar links are to the animated GIF versions, click enhanced near the upper left corner to see if you've got the mystery plugin needed. It's a little nicer when it works. Composite shows what reflects at all altitudes.
Text RADAR Composite RADAR base Long range RADAR