I'm working on a unit plan, with one of the lessons revolving around Julius Caesar as a military leader and what the military was like at the time in regards to weapons, armour, tactics, regulations etc...I'm looking for primary sources that may mention specific descriptions of this sort of thing so that I can include some passages.
Starting with the memoirs of Caesar would be good. The Gallic War and The Civil War
Beyond that, my late Republican knowledge is a little more scarce. Someone with more knowledge in that area could probably point you towards more sources, though you probably aren't going to find as many details as you may be hoping. The Principate is notorious for lack of detailed information in some regards, this period is the same.
But I would definitely start with Caesar and look at the descriptions that he gives of his troops. Highly propagandized, but still valuable first hand information.