I've heard of (never used) OBX. Word on the street is you get what you pay for. They have a reputation of building crappy parts, whether or not this has changed I don't know. Stock will do you just fine, unless you've turbocharged your car, then I'd get a high flow. Any muffler shop in your area should have a cat to fit your car if you don't want to pay dealer prices. And warranty issues will be easier to deal with if you buy locally.
BTW: e-bay = e-gay (It's a joke) :chuckles: