I would like to suggest my two go-to places which are Craigslist and cheapest textbooks.com. The site gives you rent, buy and even resell options for a large number of online college textbook rent/buy websites all in one convenient location. Also, most of the featured sites offer the option to buy if you decide you want to keep the book. In example, I once purchased a book that was over $100 in my bookstore for $7 on this site and that included the shipping cost! Also, as a note, if you see a really good price on the site you may want to snag it quick I have seen $30 price changes and increases over the span of just a day or two.