The ace near me has hollow tubes in aluminum, stainless steal, copper, and zinc in every "imperial" size possible. Okay a small exageration, only by 1/32" increments and a few 1/64" steps. But zero metric.
The neareat HD also had some hollow tubes, including 3/8". But they didn't call them "tubes" or "pipes". I can't remember the exact name, though. "Hollow rods" maybe? Don't ask me what, but there seems to be a huge difference between "tube", "pipe", and "rod" (or whatever it was called) when talking to people in the know (including, but not limited to, HD people). *shrug*
DO NOT use 1/2" size!!! Nothing, and I mean nothing, would fit.