According to my research, gunpowder and its individual components were a scarce commodity throughout colonial America and the AWI with the vast majority being imported. During the revolution, France, Netherlands and Spain supplied the Americans with gunpowder, saltpetre, and sulphur via shell corporations set up by Ben Franklin and others and shipped to the West Indies where smugglers brought to America. Great Britain obtained most of its sulphur from India (which it controlled through the East India Co after the F&I War). France got most of its sulphur from Sicily (a lot of volcanic activity). France had the best quality blackpowder in the world at the time thanks to Lavosier's manufacturing refinements.
Bunker Hill (Breed's Hill) battle was fought over the American's cache of 80,000 lbs of gunpowder and was ultimately the deciding factor in the Rebels loss (lack of gunpowder - "Don't shoot 'till you see the whites of their eyes") though it was a pyrhic victory for the Redcoats.
The Continental Congress was very aware of the American's lack of gunpowder manufacturing and raw materials and Patriot printing presses printed fliers on how to make gunpowder and where to obtain the raw materials. Charcoal was pretty easy considering the vast forests, saltpeter was mined from bird roosts in the wild and from church steeples as well as collected from human urine. Suplhur it seems was the most difficult coming mostly from local sulphur springs and distillation of the water
Even after the war, to encourage domestic gunpowder production, Congress (with Sec Treas A. Hamilton's suggestion) placed a 10 percent tax on imported gunpowder but exempted the duty on the import of saltpeter and sulphur.