Book prices vary widely. They can range from $15 (smaller reading books) to up to $200 (large textbooks). There are typically new books each quarter, unless you are taking a class in series (e.g. Math 30 series, Chem 20 series). Classes in series tend to use the same books.
You can purchase them new or used at places like the UCLA store (on campus or online), Amazon, half.com (Ebay owned), and Barnes and Noble (online). You can also purchase it from other students (check your Class' Facebook page for postings).
I would recommend to purchase them used since they are cheaper. There were some quarters where I didn't have to buy any books because my upperclassmen friends let me borrow theirs. So, get to know older classmates since many are willing to sell it to you cheaper/borrow it for free.
In regards to renting, I've never done it before. One site I know of is chegg.com.