The CU Libraries do not ordinarily purchase course textbooks, but professors sometimes put copies on reserve. To access an item that is on Reserve for your class, go to the Libraries' Catalog and select "Course Reserve."

If you are required to buy (or rent) a textbook, you can search for your books through the University Bookstore. 

If your professor is using one or more online, open access textbooks, there will most likely be a link in Brightspace for your course.  You may also search for open access textbooks at the Massive List of Open Educational Resources (OER).  Remember, however, that you must find the exact text book (title, author, year of publication) that your professor has specified.  This is not a case of "any textbook will do."