Where can I find Vitriol?

I found the recipe for the superior swallow potion, but I'm missing one important ingredient: Vitriol. I'd guess that this is an alchemy ingredient, but I don't have a recipe for creating it. I also never saw anyone that sells it.

Where can I get some Vitriol?

Solution 1:

To get vitriol you must obtain the recipe from the a druid located in Skellige Isles near Gedyneith fast travel post; it should be somewhere on the right. There should be a alchemy logo near it, if not travel down the hill while going left.

Moreover, to be able to trade with this druid you would have to complete his quest. I believe you have to be around level 21. This quest is combined of 3 parts, again, very long but that's what you get with RPGs. Once the quest is completed you would be able to trade with him, and get the recipe.

(Sourced from http://www.gamefaqs.com/boards/702760-the-witcher-3-wild-hunt/71858977)

You're welcome!

Solution 2:

Free Recipe (guarded by lvl 1 wild dogs) is located in a burned house SE of "Harpy Feeding Ground" waypoint, exactly north of Heatherton.

Solution 3:

North from the fast travel "Garin Estate" in Novigrad, there are two bandit camps at the very end of the map, both guarded by lvl 33 soldiers. You'll find the recipe from one of them.