try thrift stores, craigslist.org, booksalefinder.com,
or cheapbooks.com to compare websites.
Search more thoroughly on the internet if you're looking for a specific book. For example, a basic search on amazon will reveal the most recent edition (whose publisher is probably paying amazon commission for prominent display). If you're not concerned about edition or format, do an advanced search, and organize the search by price from lowest to highest. You can probably find a very good copy of any book you'd like for the minimum 0.01 + 3.99 shipping.
on half.com, earlier editions of books are easier to find, though there's a higher minimum price of .75 cents. However paperbacks ship for 50 cents cheaper than amazon, and because it's a less used site, even the newer books that don't have any older editions are generally cheaper. Plus, if you want to order mulitple items, each additional item only costs 1.50 more in shipping on half.com, whereas on amazon it costs an additional $4 for every book no matter what. Expedited shipping on half.com is cheaper also-- $6 instead of amazon's $7.
abebooks.com and Alibris.com--the books can be a little cheaper or more expensive than even amazon, but they could be great potentially if you need to order multiple items-- additional items can ship for as little as .49 cents more.