Are looking to replace the exhaust with a stock one or an aftermarket?
Check out BMW of Towson, might be a little far, but they're pretty damn good if you are going strictly OEM.
On my old 325es, I just went to Midas and got an Ansa muffler (I didn't really care at that point) and it actually held up great for the last 2 years I had it before I traded it.