For lens I'm sure you can find shops selling sigma equipment in Ap Liu Street's Golden Computer Centre.
My buddy brought me to this shop which sells third party stuff like remote control, filters and lens cleaners from china. Really good prices, we're talking $25 - 45 HKD kinda cheap =)
The address is 156 Ap Liu Street, Sham Shui Po, nearest Sham Shui Po MTR station. Fu Li Dat Asia is the name.