Most people buy their books when they arrive on campus. The bookstore arranges texts by course so its real easy to get all the books you need quickly. Also, the bookstore maintains a website (
http://www.vanderbiltbookstore.com/) where you can buy your textbooks and have them hold it for you (or mail it to you if you wish).
The website gives you snippets of info on the book which you can use to look for better deals elsewhere. Beware though, for some classes you need "bundles" of things and not just the textbook.
The best way to see exactly what you'll need for the class is to email the professor if you know who you have (ie, not "Staff")