As some here know, I have kind of a camera collecting habit... most of the better deals tend to be thrift stores and flea markets.
Antique shops always have cameras but usually want unrealistically/insanely/idiotically high prices.
Yard sales can be good but I've found that many people associate "antique" with "valuable" and think they're going to get rich from Grandpa's old Argus. But, sometimes people just want some things gone. I picked up an old Bogen enlarger for a buck about a while back, and converted it to a copy stand.
If you catch Goodwill when they have sales, you can sometimes get them for a buck or less.
If you're not concerned with functionality, Ebay is an excellent source. In fact, sometimes you can get package deals on several cameras "as is" because the seller is too lazy to test them (or doesn't know how).
Also, I've recieved a lot of responses by advertising on Craigslist in the wanted section. But, a lot of people see you as an opportunity to get their money back from cameras they couldn't hardly give away. (mostly cheap/obselete digital cameras and video cameras) Sometimes you hit paydirt though, and come across someone wanting to just give a camera a new home.