I have seen it at Trader Joes, Wild Oats and Rite Aid, possibly then Drugstore.com carries it then too if you need to mail for it.
I stock up when I see it on sale. Especially great for times like a long drive in the summer desert heat for me as well. Its like an instant balance.
So far the best price I have purchased Emer'gen-C for is $6.99 at Wild Oats for box of 36 packets. I have seen asking prices for as high as $11.99.
It's in a powder form. So many flavors to choose from. Any box, any flavor will do. It is described as a "Vitamin C fizzing drink mix". I mix it with a little water. I use it the morning after the liver flushing. I do it first thing, prior to finishing off with the two epson salt drinks.