Potassium nitrate is available for purchase in small amounts at most garden supply stores, according to About.com. It is sold as stump remover and can be found at stores such as Lowe's and Home Depot.
Garden supply stores used to sell potassium nitrate under the name saltpeter, but it is harder to find under that name now, according to About.com. Commonly used to make homemade smoke bombs and fireworks, this compound is offered for sale by many online garden and home supply retailers and can also be made using ingredients found in common household items. About.com offers links that give steps to making potassium nitrate at home.