One way to obtain Negotiator is from Serrit's corpse along with his notes during the quest "The Assassins of Kings".
In Iorveth's Path, it is given as a reward for gathering the necessary items to disperse the mist of wraiths or for gathering the necessary items to complete A Matter of Life and Death.
Negotiator returns as a silver sword in The Witcher 3: Wild Hunt and comes in 2 different forms:
Negotiator (crafted) Edit
Negotiator (relic) Edit