The ways in which gladiators were recruited, trained and maintained was, for the Romans, a distinctly dishonourable business. Few citizens of any consequence got involved except as a mask for hiring a gang of street toughs, and so accounts of these matters emerge only slowly into the written record.
We know that in the earliest days of the gladiatorial munera, the gladiators were largely recruited from amongst the personal slaves of the man whose funeral rites were being celebrated. Those slaves who would previously have been earmarked for sacrifice were put to fighting each other to the death instead. Often the dying man would specify in his will which slaves were to be used as gladiators and how much money his heirs should spend on entertaining the crowd of citizens which was bound to turn up to watch. One patrician stipulated that the gladiators who fought and died at his munus should be the teenage slaves he had bought to be his homosexual lovers. The youth of the proposed victims and the manifest unfairness of the man’s will led the magistrates to overturn it. Older men were chosen to fight instead.
During Rome’s long wars of expansion through the Republic and early Imperial periods there was a ready supply of prisoners of war. These were trained soldiers who could be pushed into the arena with little need for preparation. Nor were they particularly expensive, so those who died meant little in the way of financial loss to those staging the games. The survivors could be sold to citizens wanting gladiators or put to work on farms and in workshops.
By the middle of the 1st century AD, however, the situation had changed markedly. The demand for gladiators and arena games was as great as ever, but the ready supplies of prisoners of war were no longer available. To remedy this difficulty an entire business grew up to recruit, train and maintain the gladiators.
From "The Age of Gladiators" by Rupert Matthews