Pypes makes a stainless exhaust system that can be polished, they offer systems in 304 stainless (which can be polished) and 409, which will not hold up over time.
Magnaflow's systems are made of 409, but the mufflers are 304. I believe Stainless Works offers a 304 system that can be polished, and I'm sure there is one or two other companies that have 304 stainless systems.
John