Other than BMW? You could try some of the breakers or there's one here in washington :lol:
https://www.ebay.com/itm/BMW-Z4-M-X-Cross-Exhaust-Pipe-E85-03-09-OEM-7-836-639-/332095491126?_trksid=p2141725.m3641.l6368
Otherwise you could get something custom made or an 'upgrade' from the like of supersprint etc, which won't be cheap.
What happened to yours?