Did your course syllabus list books you need to buy?
The bookstore is where you want to visit for all of your textbook needs. Visit their online store. The bookstore will show you required book purchases for your courses and also allow you to use any vouchers or credits you have.
Sometimes, professors will link required readings in the modules of the course. Look through your modules to see what is linked. Other times, they will use the "Reading List" tool in Canvas. Look in your syllabus and the left menu of your Canvas course to see if there are readings linked there. If the professor told you to find a source at the library, look for it in Primo search.
If it is a required purchase, you can buy it through the bookstore (above) or find a copy online to purchase. Search for a "used" copy to save money. On occasion, an older digital version of a textbook may be available on the internet.