You'll need potassium and sodium nitrate. The smallest quantity I could conveniently get was 1 pound each, total $20.
You don't actually need both. Either one by itself (usually sodium nitrate) also works the same way for what we're doing. The main advantage of mixing those two salts is the lower melting point, which matters for some heat treat operations like tempering of carbon steel, but we are using it at a much higher temperature anyway so that's irrelevant.
