Local used books stores are a great find. Many times, they will carry original version of books that helps any book lover finish their library collection and at a discounted price. If you're not afraid of a little writing in the margins or small tears in the cover, these stores are great for finding those hard to find books.
Amazon is always selling books at discounted prices and allows you to pre-order books at a guaranteed low price. If the price changes between the time you place the order and the time the order is shipped, Amazon charges you the lowest price offered. On top of that, Amazon offers free shipping for orders of $35 or more as well as 4-for-3 books.
For a brand name company, Barnes and Noble offers a great collection of bargain priced books, in stores as well as online. On top of that, Barnes and Noble offers free shipping for orders of $25 or more.
Half.com is an Ebay Company offering used books for as much as 80% off sometimes more. They house a bargain bin where you can buy paper backs for $.99 or less and hardcover for $1.99 or less.
Speaking of Ebay, it too is a great website for getting quality new and used books at a pretty decent price.
AbeBooks is an online used book store similar to Half.com. You can search for rare books, paperback and hardcover, even some brand new books.