Some while back I bought some kelp tea, sold as a powder in a small tin with a tiny measuring spoon, at One World Market in Castleton. I recommend visiting the store if you like Japanese food and groceries.
Kelp tea, konbucha, 昆布茶, is a popular winter drink. Kelp reportedly thickens and darkens hair. That is what many Japanese told me, although whether "hair" meant on the scalp or everywhere, people hedged.
Serving size is one included spoonful to 100cc (about 1/3 a soda can) of hot water. It smells of kelp, tastes like soup broth, and brings back memories of Hokkaido.