Look in the bookstore on the campus of your college or university. Make a note of which textbooks are available that you need, their price and the condition they're in. Remember, if you purchase textbooks through the campus bookstore, there's a chance they might buy them back for a fraction of what you paid.
Find out if your campus bookstore has a website. You might be able to find a larger selection of textbooks on the website.
Check other online stores that might sell textbooks. Sometimes, your professor might refer you to an online store that sells the book to help you find it for a cheaper price than the campus bookstore is offering.
Verify that you're comparing the correct textbooks. Check the ISBN number for the books, as well as the edition. Textbooks tend to come out with multiple editions, so you need to make sure you're purchasing the correct one for your class.
Factor in the condition of the textbooks you're comparing, the price and whether or not you might be able to sell the books back once you're done with them. Choose the option that makes the most sense for you based on the criteria that you care about most. For example, if you don't have a lot of money now, you might only care about which textbook has the cheapest upfront price.