Wade....
http://www.merriam-webster.com/dictionary/sphagnum
Incidentally, it is not always labelled with the word "sphagnum". And I do not know why. But once you've seen it and smelled it, you'll know it the next time you see & smell it. I like mosses and the way they smell. Live sphagnum growing in a bog is absolutely beautiful. Green and soft.
I have bought Exo Terra brand "Forest Moss" from "tropical Asia" at PetSmart, in a 2-pack for $8.99. That is, two dry compressed bricks that you rehydrate in water, when you're ready to use. I bought it the first time because I had never seen it there. I plan to communicate to them to please keep it in stock. I have 17 snakes, 2008's, and for my purposes, is a reasonable quantity, in a reasonable form, at a reasonable price. (Apartment living, etc.) It is not sphagnum, but it smells good, anyway.
I keep at least two hides in my vivs, sometimes three. And one is always a moist, mossy hide. Moisture
just at the time of shed is good, but moisture any time the snake wants it is even better. A snakes shed is a cumulative process of everything since his last shed.
I'm sure some of the larger scale breeders who have already posted have excellent advice on most quantity for lowest price.