Nethermine Flayer Hides are skinned from the Nethermine Flayers located within the Netherwing mines. There are two entrances into the mine, though both entrance paths join together in the mines. While in the mines keep your eyes peeled for glowing Netherwing eggs
Entrance 1: 65.2 90.2
Entrance 2: 62.9 87.9
The Flayers are located towards the back of the mine, when you enter, follow the mine cart tracks on the left and you will eventually come to a junction in the tracks, you can stay left to find a few flayers, or go right and follow the track that loops north, into the main area full of dead miners and flayers.
Also, kill the blood of draenor mobs as they drop encrusted objects that can be opened that can sometimes contain flayer hides, netherdust pollen, and other stuff that's mostly useless.
For skinners the drop rate of the hide is quite low, my average rate was roughly 12-18%, sometimes in the 20's if I was lucky. On the plus side flayers drop plenty of Netherwing crystals and the occasional Netherwing egg along with cloth and coin.