go to Yahoo! Auctions HK, best way to find stuff in HK.. (use online translator for help navigating) Then just send message to the seller, all the transactions are done hand to hand in the MTR (subway)
Otherwise, try the computer mall in Wanchai (google it) or the one in Mongkok (same thing, google) it's near a shop arcade where you can find the headphones usually seen on this forum, for very good price I believe (don't mention fakes, they are not very popular here, even hard to find)