In reply to TonyG:
I bought plenty last June at a place just in the main bazaar at the Old Fort Rd end, up a few stairs, pretty much underneath the rooftop pizza place, opposite a popular ATM/bank. They were Korean, decent quality, lasted well and were around eight dollars each. He bought back the full ones at the end of the trip too.
You see plenty of trekking shops with canisters in the window but they're mostly for show, or empty.