I know a couple of good pearl vendors in Bangkok:
1) Lisa Pearl at MBK mall, phone (+66)-081-7007366)
2) The pearl store at Ja-Tu-Jark market, phone (+66)-084-7173543
There are other companies selling/farming pearls in Thailand:
http://www.rangyaipearl.com/ http://www.phuketpearlfactory.com/
Good Luck!
Tanakarn