Until the A Small Victory event, Barnar was located in Stonehold. There is only a note by him left.
Lore & Dialog
Barnar the Tinker tells you, "I am Barnar the Tinker. Mechanisms and woodworking are my specialty. If you bring me the heart of a wood golem, I can make a crossbow stock and mechanism for you.".
You give Barnar the Tinker Wood Heart.
Barnar the Tinker tells you, "Here is a stock and mechanism for your crossbow."
Barnar the Tinker gives you Crossbow Stock.