Always dropped by Bark blamish snails (level 15) in the Mort Myre swamps and in the Haunted Woods.
Uses:
It can be crafted in a snelm (snail helmet) at level 15 crafting.
Notes:
You need to have completed the Priest in Peril Quest to access the Haunted Woods and the Mort Myre Swamp (needs also to have at least started the Nature Spirit quest). Note there is only the round bark snail + shell + snelm - not a pointed one.