potassium nitrate is used by butchers and that's what for you could buy it for. remember it well, it came in 250 gramm bags and cost 3 german mark.
this hole explosive misuse is pointless. if you want to build a bomb anyone 18 years old can buy a fireworks batterie. for example this one https://www.pyroland.de/Feuerwerkbatterien
https://www.pyroland.de/Conquisator-16-Schuss-Feuerwerk-Batterie
having 395 gramm blackpowder cost 26 euro = 1 kg blackpowder cost 65 euro

1 kg swiss blackpowder No. 1 cost now 103 euro

so what will you buy when being a price-conscious terrorist? and you don't look for the clean burn of your load.
its simply stupid.

and btw, the war in ukraine will flooded the black market in europe for decades with anything from handgrenades to bazooka.