Unless otherwise noted in the textbook list for your courses, the following sites are available for purchasing and renting textbooks.

Web sites where you may find textbooks: 
www.amazon.com
www.barnesandnoble.com
www.allbookstores.com
www.abebooks.com                         Usually has these for a reduced rate
www.alibris.com
www.bookfinder.com
www.textbooks.com                         Sells and rents used textbooks
TextBook Rush (http://www.textbookrush.com/),