Eisenmann Exhaust for our Compacts. This guy is in the UK, but his auction states that he ships worldwide. I would jump on this madd hard if I had the loot to spare. Of course if you have the M rear bumper, you'd need the 325Ti diffuser insert instead of the standard 318Ti insert to make the exhaust fit.
But I actually bought the exhaust mentioned here for my project car for Total BMW magazine. Got it today and wow, what a beast. Should give some nice notes out of the rear after alterations