Well speaking for Europe the Brico stores has a rod section: you'll find tubes or rods starting from 2mm diam up to 10-12 mm diam, also some bars and "L" or "H" extruded profiles. Brass or Alu.
I bought 30mm diam alu from a pro supplier: he's more than happy to sell to me a 3mtrs (approx 35 eur) long "rod" and mostly "cash
", I bought also a 16mmx3000mm rod. Bigger diam are quite expencive (if you buy a whole 3 or 6 mtrs rod) and you'll find a lot of wastes for free in a friend's metal workshop. I know several guys running a metal factory but hadn't the time yet to begging alloy
I take any year brass hex rods at Milan's Hobby&Model show from an hardstore-vendor , from 1mm up to 4mm "diam"