I've had a very hard time finding potassium nitrate in my area. There is nothing that even resembles stump remover over here, the hardware stores tells me I need a license to buy that stuff, and it's usually not available commercially. So I headed down to the pharmacy and asked for salt petre, the old lady there knows about the product, but it's been quite a few years since she's seen it around, she told me that it's not available anymore because you could make napalms with that sort of stuff. Then she looked through her computer and found a product that's pure KNO3, it's name is Potassium Nitrate so I bought a tub. It's pretty good stuff, pure white and very fine grained, it dissolves instantly.
So head down to your pharmacy and ask them to look through their product listing on their computers, should find something that would work for you.
HTH