95 hours into this game and I still haven't found it. I started farming those Ice Trolls in Skellige but they only drop hides and some other mat. Is it part of a main quest or Witcher contract that I missed? Please help!
I know that the mutagen doesn't come from a contract. Try rock trolls (as mentioned above) or it may drop from a particular troll that you fight during one of the major quest lines in Skellige. As for the Decoction recipie, it's found at random in the environment - can't give you any tips there. Definitely worth looking for though, the stats are really good on it.