New: www.bamm.com
If you buy their $5 discount card, you get an additional 10% off every order, and most their books are significantly under list to start with. Usually the cheapest prices overall I can find for new textbooks, but if you go to google and do a search for textbook price comparison, you may get a site that will direct you to a cheaper place for a specific book.
Used: www.half.com & www.amazon.com
Unless there is a significant change, the previous edition may suffice for your class, and especially on half.com, previous editions are often $5-10, whereas new ones are about half price. Several of my classmates did this (I wish I had thought of it before buying my books) and they aren't missing anything that the current edition contains.
