If you find somewhere please post back.
About 8/9 years ago I drove all around Pattaya, Siracha and down to Sattahip trying to find a source.
Ended up having to buy loads of the small bags at 40/50 baht a bag from a place in Ang Sila. Even the person I used to relay the garden used the same supplier.
No-one, that I could find, could offer a bulk alternative back then.