Hi! If you want something on HK Island,
there is an art supply store called "Artland."
It is located in Lockhart Center (3rd Floor),
on the border of Wanchai and Causeway Bay.
The store has a wide selection of art supplies
and craft supplies. My guess is that the store
is approx 1500-2000 sq ft including a small
gallery. Lots of good quality stuff---imported
brands and local ones too. Hope this helps!
PS: Sham Shui Po is lots of fun!