Let me look in my guidebook for you.
In vendors that sell ingredients, in West Weald, and in a concentration is found southwest of Miscarand.
Go to The Main Ingredent in the Imperial City
Camoran's Paradise should be the place that's easiest to find it, but chances are that you can't get there at the moment. If you still need more than is available in the other places listed, you can check in the garden at the Arcane University inner area, there should be 4 (I think it was 4) Nightshade plants in there.
Wherever they said. And don't forgot to eat it.
^ It has the effects of Acid. It's really good to eat it.