Yes, I know posting a “recipe” for salt is lame, but not as lame as paying $3 for a tiny jar of “Popcorn Salt.”
Popcorn Salt is just Kosher Salt that’s been finely ground to stick to the popcorn better. You can make your own, pretty much for free.
Kosher Salt can be easily ground in a mortar and pestle.
It is also possible to do in a food processor with a metal blade although it will severely scratch the work bowl and dull the blade.
Using a stone mortal and pestle, put the desired amount of salt in the mortar, and grind with the pestle until really fine.
If you don’t have one, buy one. They’re about $10 at any Asian grocery store and are hugely useful.
Note that the reason I specify Kosher Salt is because it contains no additives like iodine, that can effect the flavor of the food.