No offense, but that's the wrong answer. Best way to buy a phone from HK is Ebay. I buy just about all my phones there now at present. X1387 and phonesourceusa are two reliable Ebay sellers. In the former case, they ship from Hong Kong and quite a few times now I've had the phone NEXT FREAKIN' DAY. In almost all cases, it's 2 day shipment. VERY reliable and never had an issue with them.

If you're *IN* HK, there's a cellphone mall in Mong Kok. Take the MTR to Mong Kok and take exit D2. I wish I could remember which direction, it's towards the Wang On building IIRC. Chockablock full of mobiles. Shop around though. Bargain and DO NOT buy from the first place you visit. I've purchased phones for $800HKD lower than the first price I encountered simply because I asked around... the gweilo tax is very apparent here.