What's your budget? You can find amazing deals on used catback exhausts (assuming you're looking for a CBE). Otherwise, there are a plethora of reputable exhaust companies for a new one such as Tomei, Inividia, Cobb, Borla, Apexi, ETS, Greddy, Grimmspeed, HKS, and Injen.
I'd personally suggest Nasioc for used CBE's since that's where I've found countless deals on parts of every aspect of the STi.