I purchased this spell today in Shar Vahl from Khura Ahmih. He is upstairs in the building next to the bank in Shar Vhal. He has a pretty complete line of shaman poison and disease based spells. Indifferent to my Troll Shaman.
There also used to be a troll that sold poison spells in the Innothule swamp. He stood in the palm of a stone hand in the North central part of Innothule. Now that the trolls have returned to Grobb, I would think that this spell vendor has also returned.
I found Shar Vahl easier to access, so I have not confirmed the status of the troll spell vendor in Innothule.
Keltsett
Troll Shaman
The Tribunal (Ayonae Ro)