Check out some of the HK car magazines, they should have some companies advertised in there.
This one seems to carry a lot of BMW stuff:
http://www.smart-at.com
I looked at their site, very small selection online, but their ad in C&D HK indicates they carry H&R, Hamann, Lumma, Kerscher, AC Schnitzer, hartage...etc.
Here's another one:
http://www.robinhoodracing.com
I'm sure you get some better sources if you post this in threads in the Asia forum for the HK members.