You can order textbooks online through sites such as Chegg and Amazon.com. Both of these sites typically offer college textbooks at lower prices than college bookstores, and both also give students the option to rent certain textbooks for a much lower price than purchasing the item. Depending on your school, you may also be able to order textbooks directly through your school's bookstore.
On sites such as Chegg and Amazon.com, you can find the exact textbook you need by using the textbook's ISBN. If you don't know the ISBN, you can also find the specific textbook if you know the title, author and edition your instructor requires.
Web-based textbook sites typically offer both new and used versions of college textbooks. Used versions are less expensive, though they may have notes or highlighting added by previous owners. Most textbook merchants do not sell books that have been seriously defaced by previous owners.
If your instructor requires a book bundled with an access code for online materials, a used copy probably no longer has this item. If a previous owner already used the access code, a replacement code might be available either online or from your college's bookstore.
In addition to buying the textbook, you can also rent it for a limited period of time, usually one or two semesters. Renting typically costs a fraction of the price of buying the book, regardless of whether it is new or used.