Lots of companys make universal, performance mufflers. Most offer mild or stainless steel models. The advantage to buying a 'system' from a manufacturer is that is should be designed for the vehicle, taking into account things like fit, actual performance gain and sound. Just slaping in a cheap, "performance" muffler might not fit well, have proper airflow or may resonate annoyingly at cruising speed.
Note that more noise doesn't mean more power. A stock engine with an oversized, 3" exhaust system will make more sound but may actually make LESS power. Simply put, higher exhaust speed scavenges the cylinder and helps create more torque. If the engine airflow is increased, there should be a similar increase in exhaust efficiency but not too much. Too large a pipe will mean slower airspeed in the exhaust and less scavging effect.
I noticed this dramatically in my Mini. When I changed from a 1000cc engine to a 1380, I left the stock, 1 3/8" collecter header and 1 1/2" exhaust in place for testing. The engine ran great but suffered a bit at high RPM. When I finished the job, I had installed the higher flow 1 7/8" collector header into a 2 " exhaust and I had lost all my bottom end power. Where I could originally pull away in 3rd gear from 1000 RPM, it would just sputter and complain. The car ultimately makes more power but is less driveable day-to-day.
I'm kinda long-winded this morning.
It all depends on your budget and expectations.
