your options are pretty much anything thats no bigger than 2.5"...
rcm has an afternarket muffler on his stock exhaust system but i believe he plans on upgrading the piping
i have installed the viggen exhaust which is either 2.25 or 2.5" piping and i am very hapy with it, i just need to have the tip relocated
also, you can purchase saab sport exhaust which is 2.5" with no resonator and ive heard it sounds real good.
good luck with ur exhaust and let us know if you plan on doing any upgrades to you NA