calculus
http://www.fluther.com/disc/1688/best-place-online-to-buy-used-textbooks/
Amazon or Half.com
www.abe.com
http://betterworld.com is pretty good, and the money goes to a good cause.
abebooks.com bingo books to be specific. http://www.abebooks.com/servlet/SellerInfoPL?vci=1616130