Potassium nitrate (also known as saltpeter) is a main ingredient in some brands of fertilizer and tree stump removers. It's also often used in fireworks and to make smoke bombs. Potassium nitrate isn't gun powder. Rather, it's a component of it. If you're looking for potassium nitrate to make your own pyrotechnics or conduct a science experiment with it, it's relatively easy to find.
Visit your local garden supply store and look for potassium nitrate by its most common alternative name--saltpeter. It may also be called "nitrate of potash" or "Vesta powder." It's also a main ingredient in Spectracide Stump Remover or Grant's Stump Remover.
Visit the "Business and Industrial" category on the eBay website (see Resources). You can purchase potassium nitrate online from several different sellers.
Visit the The Science Company website (see Resources) and purchase potassium nitrate there. Be advised the company does not accept returns, even if the containers are unopened.
Visit the AmericaRx.com website (see Resources) to purchase Humco SaltPetre Powder (potassium nitrate).