Hong Kong uses the UK system, so G/F is the bottom floor, and 1/F is the storey above that.
198 Wan Chai Road. The mall is shaped like a figure 8. Go to the right hand loop on the 1F. It's close to a store that sells snacks. On 2F, go to the left hand loop (Magic Pro HK, but their website doesn't list everything). SW corner of the building.
The 2F store has a lot more decks available, but the price might be slightly higher than the store on the 1F. There will be some difference in the stock available.
Another one I haven't been to in a long time is in Mongkok. Somewhere on Nathan Road (East side of the road). The building is between Mongkok MTR Exit E2 and Dundas Street. When looking into the building from the street, there are stores selling watches (Breitling, Seiko etc) on the right hand side, and a pair of escalators leading to the basement and upstairs on the left hand side. The store is upstairs, but I can't remember which floor. I believe it was on the East side of the building,
There probably are more out there that I haven't seen.