Practically anything with SMD LEDs (little yellow squares mouned flush to a green PCB) will be bright enough for night hiking. Avoid the flashlights with loads of 5mm LEDs in a big cluster (clear coloured plastic bulby things) as they tend to be overly diffuse and short range.
Torches are incredibly easy to find, try the little computer centre by the corner of Possession St and Queens Rd, up stairs in this building. Failing that, many of the little computer stores will have them, or 298 Computer Mall in Wan Chai. You can get small ones for maybe $45 that will boost a single AA to drive a LED plenty bright for hiking. If you want a zillion options on street stalls, Ap Lui St in SSP.
Generally though, I would only use a hand carried flashlight as a back up for hiking, you're much better off with something with a head strap. If you want a good one, look at the outdoor stores- Black Diamond and Petzl are the standard brands for these things. The stores are Pro-Trek in Sheung Wan, or RC Outfitters (CWB) or Chamonix (Mongkok).
Edit: There's a hole in the wall place that sells them near you. It's one of these places, but unfortunately the Streetview photo must have been taken before it opened up in the morning. There's a small table with LED flashlights and cheap binoculars just out on the sidewalk.
Edit 2: Decalthon has incredibly cheap head mounted flashlights for $40. Take 2-3 AAA batteries. Almost like children's toys with a LED package mounted into a siliconised strap mount, but they are remarkably good value for a hiking torch. I'd be fine with them for a short and safe night hike if I couldn't find my regular flashlights, as long as I had my phone for a backup.