You used to be able to buy potassium nitrate as saltpeter in many garden supply stores. While it is difficult to find saltpeter, you can still purchase potassium nitrate, which is used to make smoke bombs and certain other fireworks, as 'stump remover'. In the United States, you can find it at Lowes or Home Depot, among other places. Look for the Spectracide brand in those stores near the insecticides. If you can't find potassium nitrate, you may be able to order it online, plus it's a chemical you can make it yourself.