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 buy all my Marou chocolate with my own money.