Big box retailers like WalMart and Costco have eye centers as do mall outlets like LensCrafters. The perk with these outlets is that you can get your eye exam from an ECP, as well as the lenses and a fitting with what can be lower prices. This comes with volume purchasing on their part from the vender and no shipping and handling.
If you live outside major shopping areas though, keep in mind the price of gas and how far you have to go to reach this service. With replacement lenses you most likely can have the prescription filled immediately at these sellers and you have a real, live person to chat with and hold accountable. Some people receive a lot of comfort from the fact that they can actually see and talk with their lens seller.
There are good reasons to buy your contacts from your eye doctor. After checking out the other options, you may find that with the eye exam, fitting and lens purchase you actually get a lower cost for the lenses, particularly if you buy enough to last the year. Again, keep in mind that your insurance plan may determine where and from whom you must purchase the lenses in ordered to be reimbursed.