Depending on how much time and willingness you have to sell books back renting can be your best bet. ALL of the above sites offer renting options on some of their books as well) If you have more advice please comment so we don't have to repost on Facebook again.
Might be my overall FAVORITE) Type in the class and section and it tells you all the books you need and tons of options for renting. Really cool and fast.
One of the most popular sites to use for renting books. They plant a tree for every book that is rented which is cool. Can also get step by step answers to problems and other cool things
Couple people mentioned they were easy to use and good prices. I rented one from them for this semester.