I only found 2 places when I was looking at little while back when I was trying to decide what exhaust options to take.
Piper do one, well technically they do 2 one to fit their system, it's a sleeve fitment so may aslo fit ktecs ultra system or milltek, but you may need to cut it as I'm not sure on what lengts ktec or milteks are, so cant guarantee what systems that will fit, and they do one to fit standard system which the olive fitment.
Ktec also do one for their ultra system, again this is a sleeve fitment so not sure exactly what other exhausts it'll fit that are sleeved systems but may fit or may need some modding somewhere.
It just depends on what system you have currently or are looking to get. The piper one that is designed to fit standard system would probably be the best bet as that'll fit most other systems that are cat back. Think it's about £130 plus postage
But they're the only 2 options I found anyway. Or could just get a decent custom fitting centre to make one?