Are you talking about the AKC Canine Good Citizen test / title?
The AKC's website has a page with information about the test and its purpose, as well as testing requirements and a place to buy CGC stuff after you get the title, like patches or bandanas with the logo. You can find that site here - http://www.akc.org/events/cgc/index.cfm
The way most people get their CGC is by finding a local training club or dog school that is offering the test, either as part of their training program or as a separate test that is open to the public.
My local kennel club for example, GKC, recently had a "Fun Day", which is their way of wooing new members and letting people have a try at different kinds of activities they offer, such as Rally Obedience and such. They also offered CGC and TDI testing to participants, so all you had to do to get tested, was show up at the event, fill in the paperwork, pay your fee, and take (and pass) the test.