I did exactly that. I cut out the stock muffler and welded on a 2 1/4" double offset Borla universal muffler. It's much cheaper than a whole system and you get the same result. It sounds nice plus I've notice a SLIGHT difference in performance and gas mileage.

Here is the best site for cheap exhaust parts:

http://www.exhaustproslinc.com/borstainstee.html