You buy your subscription at any store that carries them. It is usually just a card that I believe comes with a code. Or you can but it right from Xbox with a credit card under "Manage Account. " Though you have to make sure that your status is Gold.
The easiest way to buy an X-Box live subscription is to go to any retailer that sells the X-Box console itself. You can also buy a subscription online directly from Microsoft, sign into your account and follow the directions.