You can always call up Wing Shing in HK first to check out the prices. There are a couple of reputable one but in terms of reliable big shop and convenience of location, Wing Shing has my pick. I think it's the tourist fright that I wouldn't dare to check out the smaller shops, especially those that makes you feel like you're walking into a shop in Lucky Plaza. I'm sure some people would have pleasant experience with some of these shops but I prefer the "Cathay Photo" type of shops. Just personal preference. IIRC the shop is right outside of Mongkok MTR Exit D2 along Argyle Street.
I did check out price.com.hk and as mentioned, might be good for reference. Might be tough to hunt dwn some of the shops even though the prices might be better. The difference shouldn't be huge unless it's water goods.