Mufflers are a very personal thing. It depends on what kind of sound you want.
If you want just a nice mellow note, close to stock, just go to your local muffler shop and tell them and they'll likely put a standard muffler on.
If you want a little more meat to your exhaust sound, try a generic turbo style muffler from your local muffler shop.
If you want annoying loud and resonating...try a flowmaster.