Another place to look is recycling yards. A family friend used to go and bring huge boxes of books on pretty much whatever subject matter you would be interested in - photo books of WW2, Soviet propaganda literature, birding guides, etc. Plus, they are usually pretty beat up, so you never feel that bad about cutting them up. I think that they just get sets of books from people who do not want them any more, so they can't cost that much (if anything)!