Most antique stores, especially rural ones, have a drawer or two full of old photographs somewhere. Finding one that really appeals to you might not be easy, but of course that's the challenge. The list of potential sources is endless. And there's no rule that says collectable pictures have to be old, so don't forget photography schools. Purchasing a picture that you think is dirt cheap could improve a struggling student's diet for weeks.
Most schools have ongoing or intermittent exhibits.
Check local photography clubs too. Once you start looking, you'll be amazed at how much there is to find. Once you start looking, you'll be amazed at how much there is to find. If you're a photographer yourself, trading work is a good way to acquire pictures for your own collection. One photographer we know here at the magazine trades his prints to friends in return for boxes of photo paper.