I use popcorn salt for my icing as well since it is much finer. Our store has it with all the other seasonings and regular salt.. not in the popcorn aisle... it took me forever to find it the first time. LOL
So look both places if you don't see it with the popcorn.
If the only difference is that the grain is finer for popcorn salt, why not save yourself some money and toss some regular salt in your food processor? My husband does this with brown sugar for some of the BBQ rubs that he makes and it works great! He will usually do up a bunch at a time and store it in a separate container. Just a thought!