This is a first. Hopefully these will be easily obtained through methods in-game and will not require an out of game purchase. My best guess is that these will be obtainable through Greatfather Winter during Winter Veil. The only thing that would make these bags better is if we were able to mail them cross realm and weren't unique. Cya later Netherweave Bag.
Other items added in Patch 5.4 with Greatfather in the name includes:
The item appears to be unique so I would wave goodbye to the Netherweave Bag quite yet!
Totally didn't miss that over the excitement of a BtA bag being implemented. Crap.