david - i go to the stationary shop like what jon yeap said. It cost about RM 2.00 for a whole box of blackboard chalk, I put them in a ziploc back, smash them until they become powdery. And then put them into a chalk sock (bout RM20) can be found in mountain climbing shops OR the economical way, is put it into another ziploc bag.

mountain climbing shops sells chalk blocks as well but for RM18.00 per block, I'd stick with the blackboard chalk.