Spectracide stump remover is pure KNO3 (potassium nitrate). Most carry garden sulfur for plants that like higher acidity/lower ph (like blueberries). I make my own charcoal. I've given up on the sulfur in black powder. As long as I have percussion caps and primers, I don't need it in my powder.
*Sulfur and KNO3 is cheaper to buy on Amazon. The more you buy, the less per pound. I recently looked at sulfer powder and you can get 20 lb for $26 with free shipping. Sulfur is an element, and doesn't go bad AFAIK. I might pick up a 20 lb bag and just stow it away. That's probably enough to last a lifetime.