Or do I really have to go to BKK for this...?

I can only think of one place but their range is not extensive ( across the street and down a bit from the Post office in thong sala) . You should try asking the silver jewelry shop in thong sala, just past Becks beer bar. It looks like they make their own jewelry (I've always meant to go in and have a look but never got around to it) and see where they get their supplies

I know the jewelry shop, next time I'll ask her about the supplies. Thanks SBK.