I have a deep and abiding fondness for a good used book store. Not least because they often don't know what they have when it comes to RPG books.
One of my favorites is Half Price Books. They're everywhere, and where we live, I get to see a continual stream of good gaming books - I keep finding obscure indie games there. Of course, there are three locations that I drive within a few blocks of daily because of my commute.
If I lived in Portland, I would go to Powell's more often - but their RPG section has been dwindling of late. The best find I saw there last time was a copy of Amber at a very reasonable price (especially given its $105 used price tag on Amazon at the moment).
Amazon often has used copies of books available, as well - usually through third-party sellers.
It's how I got my copy of Burning Empires, which was my introduction to the Burning Wheel system (which now includes the award-winning Mouse Guard game, as well).
A few weeks ago, my friend Wade took Stephanie and I to his local Half Price, and they had Weapons of the Gods in stock, as well as a lot of Earthdawn, 2nd Edition - two games I'd never picked up for a variety of reasons.
I'm always both glad and a little sad to find awesome things like this in used book stores.
Glad because it allows me to fill in gaps in my collection or buy that game I wanted but never saw in stores. And it's usually relatively cheap, too!
Sad because it means that someone put that book down. Whether it was because they moved to a new edition, or just walked away from the game (or the hobby), there is one less person enjoying that book.