Try here:
http://goo.gl/maps/pyrxn
They cut PMMA (Acrylic) and other materials per your requirements very cheap. Not sure if they have what you are looking for, but they have acrylic in different colors, PE, etc
Usually you show up, tell them the measurements you need, pay a hundred dollars or less, come back after 20 minutes and walk away with your board.
Samples of all the material in the shop.
Web site here:
http://www.thesun.com.hk/
PS: I also just saw a second hand office furniture dealer in Lai Chi Kok selling white boards. It's a shop on Castle Peak Road. Out the MTR then walk up to Castle Peak road and turn right towards the direction of Caritas Hospital. The shop is on the right side of the road.
Website here:
http://www.2handshop.com.hk/page3541.html?lang=EN