The point of my original question was not to imply that labor-management relations exist like they did prior to the New Deal. I was just curious on who these security forces were.
In places like Minneapolis during the Teamsters’ Strike of 1934, hired goons were also deputized by the police department and the sheriff. Not only were they protected from prosecution, their headbusting actions were often legally sanctified. Good book to check out is Teamster Rebellion by Farrell Dobbs.
As for BobT’s OP, basically the companies themselves or agencies like Pinkerton’s went out and actively recruited - advertising, word-of-mouth, etc. During periods of labor unrest the recruiting got even more aggressive, often with a hard patriotic edge to it as well. The local and county police had a hand in fielding recruits as well.
IIRC, one scene in Matewan showed a fresh recruit to the goon squad quoting an advertisement verbatim with the others making fun of him for falling for the line. (Kick-ass movie, BTW.)