Ebay have a genuine BMW chrome tail pipe for about £35, I paid about £12 off e bay for a none genuine part,
fits E90 OK, for Z3 make sure if getting one of these it has a hole to thread a bolt and nut through as the spring clip type have an habit of flying off. It gives a slight rasp to the exhaust note as bigger than the existing pipe, I quite like the result sounds a bit like a 60's british 4 cyl roadster.