What are looking for in an aftermarket exhaust?
Do you want a full exhaust that includes header, pipe, and muffler?
Do you want to run an exhaust snorkel as well?
Your best bet is to hit up some Arctic Cat dealers and make some inquiries.
Look into Supertrapp, FMF, Yoshimura, and HMF as they provide some "slip on" mufflers that will fit your pipe diameter.
HTH