I continue to be amazed by BookFinder. When I need to buy a book, that’s where I start; and usually end up buying a cheap second-hand copy, which beats prices of new books even with shipping costs included.
It’s not a bookstore itself; it merely searches the databases of tons of booksellers around the globe, and tries to find the cheapest solution (both among new and second hand copies of the book you’re looking for).
Disclaimer: I’m in no way affiliated with the site, I just thought it might be useful for whoever is reading this. It has been, at least, for me.