Bimmerhaus is in broomfield and are top notch. Bob tunnel is a owner and he and his wife have 20 something national championships in scca solo autoX. I had had Jim instal my ohlin coilovers there.
Another option could be scr performance up in lovelend. Mark Bear is an owner there and also a successful driver and supports the BMW racing community. The last that I know of is 303motersports. They do European I believe but I have not had direct experience with them I have just herd good things. Hope this helps.
2011 BMW 135i DCT M Sport Coral Red Tint, Apex ARC8, 235/265 Star Specs, Ohlins Coilovers w/ Swift springs, M3 front bits, Vorshlag camber plates, stud conversion, spacers