Usually I look for used books either from the Amazon.com marketplace, Half.com, or Ebay. There isn't really any one website that has the best prices, you kind of have to shop around for books that sound like they're in decent condition and have a good price. However, last semester I used a website called Chegg.com. You can rent your books from them for a semester, quarter, or longer for about half the price of the new textbook. It worked out really well for me. They're in good condition and when you're finished with them you pack them up, put the return address sticker on the box (you don't have to pay shipping back), and send them back to Chegg. It's a great way to save money but sometimes it is cheaper to just buy the books used so like I said, you have to check around for prices.