Do you need to have a prescription to purchase Contact Lenses in the U.S.?

Yes and No...

To purchase Contact Lenses from a U.S. based contact lens supplier or company you will need a valid prescription. Essentially, due to the "Contact Lens Rule", Contact lenses are classified as medical devices by the U.S. Food and Drug Administration (FDA) and cannot be bought legally in the United States without a valid prescription.

However, as someone based in the U.S., if you know the contact lenses you need, you can order contact lenses from Canada or the U.K., since it is not illegal to purchase contacts without doctor's prescription in Canada or the U.K..

You can buy contact lenses from VisionDirect.co.uk without a U.S. prescription (10% OFF coupon enter "SURPRISE").

To buy your contact lenses from Canada and save money check out Vision Pros at http://www.visionpros.com.

To buy contacts in the US you need to have a valid prescription within the last year.
However if you are purchasing contacts online you can obviously sumbit an older prescription without anyone needing to validate it. This is technically illegal therefore not recommended
It would still be advisable to get annual check ups for the health of your eye as well for possible prescription updates.
Other countries have more relaxed laws such as Canada and Australia where you can purchase prescription contact lenses with an older prescription.