I managed to get it eventually, though I cant recall what troll I got it from. I did suffer from the same problem you do though. Was probably looking for this goddamn mutagen for 100h or so.
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.
I found the mutagen from three rock trolls that Lambert and I killed in Kaer Morhen. The recipe I bought off a random herbalist somewhere I don't remember.