The primary source material Shakespeare used in the composition of Julius Caesar was Lives of the Noble Greeks and Romans by the Greek historian Plutarch. Lives was translated into French by writer Jacques Amyot in the early 1560’s. Thomas North then translated it into English, with his first edition appearing in the late 1570’s.
Shakespeare used North’s translations, particularly those sections on Julius Caesar, Antony, and Brutus.
He would later revisit Plutarch’s Lives for Antony and Cleopatra, and Coriolanus.
This is not to say, of course, that you can depend upon the Bard for historical accuracy (as we know from earlier in the project)…