www.abebooks, already mentioned, is good, but what I think is even better is www.addall.com, which is a huge umbrella type site, containing the catalogues of abebooks, as well as other similar sites such as alibris. They have details of new and used books in thousands of shops all over the world (mainly the English-speaking world). I live in England and once found a book I was desperately searching for in the back of beyond in Alaska using Addall. It never fails to be helpful.