If you want to buy something new or second hand or just take a look around go to Champagne Court which is on Kimberley Rd. (which is perpendicular to Nathan Rd. in TST). Most "high end" cameras (Leica, Rolleis, Hasselblad, etc.) command a premium in HK as it's a collectors market but they have some of the best prices for things such as Voigtlander lenses (for Nikon or Leica Mount).
There are still plenty of shops in Stanley Rd. but the prices are a bit higher as it's a laowai (foreigner) area. But there are a couple of places for decent quality film processing if you need something done quickly.