Where can I find my textbooks for my classes?
Your professors are supposed to let the Cal Poly Mustang Shop know which textbooks are required, and the texts should be listed for purchase here. For some courses the University Shop also provides cheaper access and you should look on each Canvas Class shell to see if that is the case for your class.
But before paying for a text you can also check the library's Course Reserves module to see if an electronic edition of a textbook is available at no cost to you. You can search the Course Reserves by instructor name or course code (e.g., PSY 303) here.