If I'm looking for a specific title or edition of a title I use ABE, Biblio, and maybe eBay. If I'm just looking for stuff to fill out my collection (an ever present need) I have the most luck with used bookstores.

Whenever we are going to travel (even just a couple hour trip) I open Google maps select my rout and then zoom in on population areas along the way and search "used bookstore." I almost always have several to pick from.