Go to the Kowloon side to a place called Mong Kok (there's a station there called Mong Kok), its the capital for all things regarding gadgets, computing, gaming and dvd/cds of the dodgey kind (if you know what i mean!) and also this is the place to get accessories for cars ranging from hi-fis, bodykits, exhausts, any serious mods and cons you cvan think of. They have TOMs and TRD and also the official dealers of HKS and Blitz, Apexis and Teins.
Ask anyone where China Travels are (the official building for getting visas to china) and you'll find yourself in the heart of car modding wonderland!