For online courses, you can purchase textbooks through the MBS Bookstore. When you enter the information about the specific class, the site will display the appropriate textbook(s) along with information on format and price.
For the classes on campus, you can look up textbook availability on the CMU Bookstore web site. Under Books - Buy/Rent, you enter the information about your course and the site will display the appropriate textbook(s), along with the information on format and price.
Although the Libraries usually do not purchase textbooks, if you would like to see if CMU Libraries has a specific book title in the collection, you can search SmartSearch.