I've seen some big chains at the Homepro; are there any other places that stock them? Heavy duty (ie bigger/thicker) chains are ideal.

Most of the roadside mom n pop type hardware shops have them, all different sizes and sold by the metre.

If you need something bigger than what home pro stocks, you must be into some serious role-playing ?

Give SCG home solutions a go, think i have seen chain in there.

South on Sukhumvit heading to Sattaheep, Past ambassador hotel,

a few doors down from the Honda Car dealership on left hand side.