You can get things online, if you like. I got my vitamins + from Meyer Hatchery. Feed stores also sell it.
Niacin can be bought at the local grocery or drugstore. Just plain niacin, nothing fancy or special. Dissolve it thoroughly in the water.
Zesty is a cutie pie!
And you're good to try to find out how to help him.
From what I have read here (and there are people with experience with this kind of problem), Zesty may have had a nutrient deficiency, or it could be a genetic problem.
Some things you can do: figure out how to avoid...