Nobody mentioned ABE yet? It’s great! And Books for Less over by the Mall of Ga, love that place.
I received an original on the Lost Dutchman Mine from Abe Books. A treasure in itself. I believe the author was Sims.
Some of the search engines have come and gone. There used to be a Dogpile-like multi-engine (one search box, many searches made) called, hmm, something obvious like www.everybookstore.com and I used that to track down a couple of really hard to find ones.