I use popcorn salt. It is a much finer grain than regular salt. If is usually found in the aisle with the popcorn, near the popcorn oil.
I found popcorn salt at Super Target on the spice isle near the other salts. Have never seen it at a grocery store though.
Just ask the store manager of your favorite grocery stores. Things like that can be ordered if they dont stock it on a normal basis.
I know a few of the managers at the store now. It's helpful to be able to ask about items you hear about of see somewhere and know you can request them.
Good Luck.
i get mine at the wal mart........ 3 3/4 oz. size.... along with all the other salts... morton popcorn salt
Ahh shucks!!! I was just there tonight. Hopefully I'll remember to check and see if they carry it next time I'm there! 
You can whiz up some regular salt in a food processor to make it finer grained - good substitute for popcorn salt. 
A whirl in the coffee grinder will do the trick. I have one that is just for spices...
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