I usually go to Amazon.com (I know he is a liberal YUCK) but I go to the Used Books section. Not owned by Bezos. I order them there and they are in brand new condition for a fraction of retail. Never been burned yet. And you can even get out of print books that way.
That's another good tip. B&N also has a used book option that I have used. I'm sure there is a kick-back to B&N and Amazon from these used book dealers, either up front or based on sales.
I do like to support the conservative outlets, though (unless they are totally out of sight, which is not the case, usually.)
Have you tried www.alibris.com? It is basically a marketing consortium for used book stores around the world - great place to find new & used books (with descriptions of quality, features, etc.) for thin green.