First of all, you need "Gallic wars" of Caesar, then "The History" of Posidonius (you need the excerpts from Athenaeus, IV, 151e-152f, 154a-c and VI, 246c-d), then "Celtic wars" of Appian, "Description of Hellas" of Pausanius - chapters about Celtic invasion in Hellas, of course "Geography" of Strabo and works of Livius and Polibius.