When buying new DVDs I check out deepdiscount.com . They are typically a low cost retailer of DVDs but every so often they have sale or offer a coupon code that helps you get rock bottom prices. If you can wait, they typically have a pretty big discount announced around thanksgiving. I know several people who wait for this sale each year to buy their DVDs.
Another site you can check out to comparison shop DVDs is dvdpricesearch.com