That exactly was my biggest fear in selling them, that the buyer wouldn't burn them in the appropriate holder, even when I had the square holders for sale. I now sell some just poured in the holder itself.
The Morton Solar Salt, I get at WalMart is always outside, in the garden section. It is never inside the store, so you might want to check outside there.
There is a co-op going on right now for the 8 and 16 oz. milk bottles. Bath salts look beautiful in these! I was using the glass bail jars, but since have changed to plastic bottles for my bath salts.
I use the little condiment cups, and I get mine from ebay. I think you could also get them at a restaraunt supply place, and I heard that Sam's Club, has them also.