You can try Chamonix Mountaineering shop. They have two shops in Mongkok. Compared to OVERLANDER, Chamonix is much cheaper, and the staff are a lot friendlier.
Aquapac, or Wytex camera case/bags are probably the most economical, but you won't get the same fit for the camera (especially when the lens is extended) as you would with a specifically made-for-the-camera waterproof case Canon makes... which could easily sell for HK$1,500 or more....