A university arts library is your best friend: they buy the expensive folios because students and researchers need them, they have rare books collections, and they usually buy pretty much anything important (depending on how well funded they are anyway), and are not sickly full of copies from the 1989 beginner's guide to the Nikon AF-ZM12312-UVXYZ with full-extra-super-duper features to make better photos.

I have found David Douglas Duncan's War Without Heroes at mine (McGill), and that's a book that was never reprinted after the original run. It's beautiful.