I have been looking for monster hair in The Witcher 3 for hours and I can't find it. Can you buy it or what monsters do I kill to find it?
Monster hair can be obtained by dismantling several items you loot from monsters, including Fiend’s Eye, Nightwraith Hair, Chort Hide, Werewolf Hide and Troll Hide.
Here's a guide about Monster Hair.
Basically Werewolf Hide is a good choice because you come across one in a Side-Quest.
It's the Wild at Heart Quest
There's also the Fiend that you come across one in a Side-Quest aswell
The Return to CrookBak Bog Quest
In this guide it also gives some good locations to farming Fiends.
Last but not least the Guide states that Skelliges Caverns hold some Trolls for you to farm.
Follow these steps and you'll have plenty monster hair. If you don't know how to dismantle just go to a Blacksmith or Armorer and go to the Dismantle option.
Usually there is the Shop, Crafting, Repair, Upgrade and Dismantle Options.
Hope I Helped