I'm level 31 and can teach several instruments. I think the main purpose is to be able to teach non-minstrels how to play instruments so they can be cool like a minstrel.
Anyway, I think I can teach 4 or 5 instruments.
The problem is the cooldown for teaching an instrument is 5 days. I wouldn't charge someone to do it but it would be first come, first serve.
Just go hit up a minstrel and see what they can offer to teach you. Most probably haven't used their skill.