Well, he was a Greek that became a Roman citizen about 46-120 AD. He was from an educated wealthy family. I would imagine that his resources would be what was available by oral tradition, scrolls, personal experience, shared public knowledge and hearsay, and what they taught in his school. He served his city in many different ways, usually as a leader, while he was alive.
His work was translated by Sir Thomas North in the 1500’s. Dryden published another translation in 1683, which was edited and republished by A. H. Clough in 1859.
I seriously doubt that we will find many original documents for his sources. He openly admits that some of the incidents within the stories he shares are a bit gossipy and at times he is not able to find additional sources, but he includes them to help make the Life more rounded, and to help distinguish the characteristics he is teaching. He even offers his opinion of them. Since we study him less for history and more for character, that is okay. =)
I should clarify that I meant that I know that we have records of the people he wrote about, but some of the incidents are not going to be in those public records.