I happen to have a 1-pound bottle of the “Spectracide Stump Remover” which happens to be pure potassium nitrate. I’m using it for another purpose but I think you can use it for this. Cost for that at local hardware store is $7.47.
https://www.lowes.com/pd/16-oz-Stump-Remover/4764059And I believe that “Hi-Yield Nitrate of Soda” (16-0-0 fertilizer) as sold at Ace Hardware is pure sodium nitrate. 4 pounds of that costs $7.59.
https://www.acehardware.com/departments/lawn-and-garden/gardening/plant-food/7299761