Don't bother buying reps in Hong Kong. You can wander the night markets and even be led up to some shady, smoke-filled rooms, but you'll still wind up overpaying for crap.
You should have no problem having reps delivered from China to HK. Shipping will take a day or two, and there are no customs fees. Some dealers may be able to reduce your shipping charges appropriately.
If you have a free day, take the train from Kowloon to Guangzhou. That's a two-hour trip, but there's a high-speed, Shenzhen-Guangzhou line in the works that will reduce travel time considerably. Unfortunately, I don't know when it opens. It might have been delayed in the aftermath of the derailment last year.
Do make arrangements with one of the trusted dealers to show you around. Otherwise, you could spend a day wandering aimlessly among mountains of crappy reps.