Its best to get your books as soon as you can so that way you will not have any trouble later on. Here are a few suggestions in addition to the ones mentioned above:
1. Look on this fourm or
http://www.asi.csupomona.edu/ if anyone has the book you are looking for. If they don't then you can create an ad if you wish.
2. Stop by the University Book Exchange store on Temple and Grand. I believe it is located in the shopping center near Jack in the Box.