Determine the form of potassium nitrate needed for your application or project. Potassium nitrate, although forming within caves as white mineral fibers, crusts and crystals, can have impurities in its natural state (marked by colors in the crystal such as red, brown, yellow or gray). In mass production settings, companies can crystallize, break down and refine potassium nitrate for sale as powder, crystals and crystal granules with or without impurities or in a solution.

Go to a home improvement or garden store such as Lowes to buy potassium nitrate for stump removal, fertilizer or non-pure potassium nitrate applications or projects. Keep in mind that although a stump removal product, such as Spectracide, has 100 percent potassium nitrate as the only ingredient in a 1 pound container, the potassium nitrate granules will likely have impurities in this form. Potassium nitrate containing fertilizers come in bags sold by the pound or as amounts covering a specific square footage.

Buy "technical grade," high quality potassium nitrate from a professional fireworks and pyrotechnics chemical supplier such as Skylighter, Inc.. Most fireworks and pyrotechnics chemical suppliers sell potassium nitrate at heavily discounted prices by the pound as granular crystals similar in consistency to white table sugar.

Purchase potassium nitrate in crystal, powder or solution form through science education focused chemical suppliers and distributors. Keep in mind that other than chemical and drug vendors, you can buy the purest forms of potassium nitrate from science education focused chemical suppliers. These suppliers not only sell chemicals used in a wide variety of experiments or applications, but also any laboratory equipment and supplies you may need for a project that uses potassium nitrate.