Gah! Don't support Blockbuster at all if you can. They were against DVDs in the beginning, and they usually don't support OAR now. They've become one of the 500-pound gorillas (along with Walmart) in the retail market. That being said, I've ordered nearly all of my discs from
DVD Empire and
DVD Planet, both of whom also support Wish Lists for DVDs you can't afford or want to buy later, as well as good pre-order support.
Deep Discount DVD has good prices on a lot of discs, as well as free shipping (only for First-Class Mail).
EDIT: I forgot to add that you should check the Software for Sale/Trade area of this forum. You'll find a lot of people willing to get rid of discs cheap, and sometimes someone has a rare or OOP disc that you've been looking to add to your collection.