It's called Flavacol, and it is dirt cheap online: http://www.amazon.com/Gold-Medal-Flavacol-Seasoning-Popcorn/dp/B000IQ0CQI
I could only find it online where a minimum order was $10. That meant that I had to buy 4 quarts of flavoured salt. At 3/4 teaspoon per batch, it's literally a lifetime supply. I've been doling it out to my friends and acquaintances for years, spreading the goodness of at-home theatre popcorn.
The best recipe for at home theatre popcorn is what I call '1.2.3.4': 1/2 cup popcorn to 3/4 teaspoon flavacol. Use just a small glug of oil, you don't need much.
Note 1: If you use that nasty ass orange coconut oil it's exactly the same as theatre popcorn. I use corn-oil and I prefer it.) Note 2: If your pan is a sticky mess at the end, you've got it too hot. If you dial the heat in right, the cleanup is stupidly easy, just a lightly oiled pan at the end.
Seriously, this recipe is so good, that I no longer use any butter (for movie nights at home, I used to add salt and butter to microwave popcorn, too much butter for anybody's good).
Also, when you work at the theater, if you are there at the right time the first layer of popcorn to come out of the popper is heavenly. Makes the rest taste like shit.
Make it at home and the whole batch tastes as good as that first layer of popcorn to come out of the hopper. The reason is steam, glorious steam. When you make popcorn in a theatre hopper, you make it at first with the lid down. When there's enough popcorn in the kettle to prevent the kernals from flying all over the place, the lid is automatically lifted, allowing the rest of the popcorn to spill out the sides.
So, in the theatre, only that first bit out of the hopper is popped into a beautiful sauna of salty flavouring, the rest of the popcorn is popped with the lid up, which allows all that steam released by the popcorn to be released.
The first bit of theatre popcorn is steamed slightly, while the rest is drier than a popcorn fart, as my mother in law is fond of saying. At home, you keep the lid on the whole time, so your entire batch is delicious steamy popcorn, just like that first bit out of the theatre-hopper.
You can buy this popcorn popper that does the job perfectly and easily: www.amazon.com/Wabash-Valley-Farms-25008-Whirley-Pop/dp/B00004SU35/ref=sr_1_2/189-8926199-7135729?ie=UTF8&qid=1392909828&sr=8-2&keywords=Wabash+popcorn
Enjoy guys! No more microwave popcorn :)