Just a couple of notes form a guy (me) who has my own "real" popcorn machine, as well as some insider information from a "previous life" working at a movie theater when I was younger.
1) THE salt is Flavacol as Jaime mentioned, however it really isn't that hard to find. I've even seen smaller sizes of it in grocery stores. The trick is to look in the phone book for "restaurant supply" or "concessions" places and give them a call. You can get it in what looks like a 1/2 gallon carton.
2) THE oil is coconut oil (also called 76 degree oil since it is solid up to 76 degrees). While not the most healthy for you, it is by far the best tasting popcorn oil there is. Second, and a little more healthy, is canola oil. This oil can be had at the same place that the Flavacol can be gotten. No since in paying for shipping if you can avoid it.
3) THE popcorn is..... well.... Really just about any popcorn with the right "expansion" ratio (which IS labeled on the bag) will do. Your more major brands tend to have fewer unpopped kernels and larger pops too. You want between 40 and 44. 40 will be on the "chewier" side, while 44 will be towards the more "brittle" side. Again, almost all popcorns fall into this range, but if you get something with a rating of 41 and want a little more crunch, next time get some 43 or 44.
4) THE "butter" topping is... actually an oil and not butter at all anymore. I would say that if you are making the "good stuff" at home, then get some real butter and melt it slowly for best results. You can buy the "buttery" oils even at places like WalMart, but again, why put cholesterol into your body unless it tastes good and I think that oil crap is just that... Crap. If you MUST have the "buttery" oil like at the theater, then you will need to find out what your favorite theater is using because there is a wide variety of oils used by different theaters. A simple stop at the concessions stand and you should be able to ask what they use and they should show you the packaging unless they are jerks about it.
Another thing that people can do is buy those "portion packs" that you see online (and you guessed it, at the same restaurant/concessions supply places). I would say start with ones made for a 4 oz popper unless you are feeding a lot of people, but even so, you can always put 2 packs together as long as the cooking container is big enough. You just put the pre-measured oil (coconut), popcorn, and Flavacol in at the start and cook away like normal. Best results will be had by using a hot kettle and letting the oil warm up first.
Best of luck... I need to go make some popcorn now!
