Look around in your area also. I know that we have one huge place that deals with strictly used books. We also have some smaller places as well. They generally sell paperbacks from a quarter to a buck. This way you can spend an hour or more there and walk out with an enormous amount and still spend less then $20. Plus, you don't have to pay shipping and handling, which can really jack the price up and make a good deal not so good.
You can always check out ebay. Often times you will find lots of books going for cheap, but you may not be interested in all the books in the lot, so, it can be hit or miss. Not sure of other websites though, all though half.com also does used books, I have only used them for books I couldn't find elsewhere.