http://www.abebooks.com/
I use them to get old, out of print books that I have wanted for years. I am ordering about $200 worth of books through the various booksellers, things like:
Richard Halliburton's Complete Book of Marvels- Anyone in the 35+ range might remember this fascinating book from school.
"Salt Wind and Gobi Dust" "And a Few Marines" by John Thomason, cracking good stories.
Quest in the Desert and Quest for the Snow Leopard- by Roy Andrew Chapman, again cracking good reads.
Also The Bolo Battalion by Gordon Shireffs, about the Phillipine resistance in WWII.
AND The Three Investigators series, much, much better than the Hardy boys ever thought to be.