Here are some other websites to get student books from:

http://www.ecampus.com This company allows the selling back of books when the student is done with them and the renting books rather than buying one outright, as well as e-book sales.

http://www.textbooks.com

http://www.directtextbooks.com This site searches over 200 websites to find the cheapest textbooks.

http://www.amazon.com

http://www.cbd.com

http://www.alibris.com

or you can go straight to the publisher's website and order from them.