I always used half. com or bigwords.com. Bigwords, will search various sites and post all the prices at each site. Also, if you know it is not a textbook you want to keep and you don't highlight, many college libraries have textbooks you can check out. I did this with some of the classes I knew I wouldn't keep the book and just renewed them through the library. If the library doesn't have it, look at inter-library loans.
HTH. Christina