I get my textbooks from BlilBook (http://www.blikbook.com). They are etextbooks, and they have a lot of educational textbooks there.

There are many e-commerce sites that sell new and used textbooks at prices lower than what a student will find at a college bookstore. Most of these businesses carry a very large inventory of textbooks that cover all academic disciplines, so students with any given major will be able to find the books they need. Many students have learned that the greatest economy in buying textbooks is to buy them used and then sell them back to the merchant or to another student when they are done with them. This practice can result in significant savings, sometimes even allowing the student to recoup most or all of a textbook's cost. In addition to saving money by buying new and used textbooks on-line, renting textbooks is also a means for paying substantially less than retail price. Rental prices are lower than used prices, with the stipulation that the textbook be returned in a timely fashion. Failure to return a rented textbook on time can incur expensive fees. Many textbook sellers offer promotional coupons, so do some research before settling on a vendor. The lowest possible price may result from the use of a coupon code. Some of the more popular sites for getting a deal on textbooks are:

Amazon Marketplace - sells used/new textbooks
Bookbyte.com - sells used/new textbooks, rents textbooks
Chegg.com - rents textbooks
eCampus.com - sells used/new textbooks, rents textbooks
Half.com - sells used/new textbooks
Textbooks.com - sells used/new textbooks, rents textbooks

Another option is to use a textbook price comparison site to search for the best deal.

BigWords.com
BookScouter.com
CampusBooks.com