Go to your school's website and search its bookstore pages. Have a list of your classes available for reference, including the course title, course number and professor's name if you know it. This is one of the most reliable ways to find textbooks assignments before courses begin. If you cannot find your textbook requirements online, call the bookstore or visit it in person.
Ask your friends if they have taken any of your classes before. Ask them what books were used and where was the cheapest place they found to buy them.
Monitor social media networks and email messages for offers to sell used textbooks to new students. The seller usually advertises in which class he used the book. Look around campus for fliers from students who are selling used textbooks.