Google Echo Photo along Hankow Road for Lenses and Bodies and original flashes. Accessible by Tsim Tsa Tsui MTR Exit C1 You'll see Isquare. It's 2 streets east. I'm sure you can find it on the map. Every hotel has them. Find a Mr Yip who you can claim to have known through a friend. His open offer price is easily the lowest but don't get your accessories from here.
For accessories, flash, sd card, trigger led lights, softboxes, original batteries, bags, straps, anything else. go to Sim City. It's accessible by Mongkok MTR station Exit E2. Exact location is at the intersection of Shantung St. and Sai Yeung choi street. Make sure you comb the place thoroughly before buying your stuff comparing prices and such. you'll be surprised how cheap the things can get.
Other places not worth looking unless you want something the above places do not sell. In that case, go look at fortress, broadway all the bigAs chain shops. Only for those elusive things.