Apart from Sailor being an obvious recommendation, I wholeheartedly suggest trying Marou products. They are a Vietnamese company who source and process their nibs locally and produce chocolate of the highest quality in an ethical manner. (See Marou Faiseurs de Chocolat)
Note: I hope neither Stephen nor Azizah – who run the popular fountain pen show “Serious Nibbage” – mind the playful use of the name of their show in this context.
Disclaimer: I’m not affiliated with Marou or any of their resellers. I acquire all my Marou chocolate by buying them with my own money.