The same is certainly true in sales and purchasing, many aspects of human resource management, and other decision-making processes of business management. There is really no profession called 'negotiator'. People study the process and skills in law, business, foreign service, and some other professional schools. In addition there are many courses in negotiation offered commercially. In each case the training is effectively aimed at taking the individual's innate skills and personal style and enhancing the effectiveness with which those are employed.
Lawyers are most likely to be used as 'hired gun' negotiators.
I must admit that in my own case and most likely in many others, people who teach negotiation are often called in to provide assistance as mentors. What troubles me is that many times the folks who employ these 'hired guns' are looking for short-term results. If you cheat me twice, shame on me. You may want to investigate negotiation's isotope, mediation. I must add, however, that most of the people I know of who make money as mediators have been educated as lawyers.