I don't know if they can help but they do have a manufacturing process for aluminium tubing possibly your size but could be for commercial application.
I use Capral Aluminium
www.capral.com.au for all my needs, al-be-it for square tubing for racking systems in my vans.
At least it is a staring point and would be good to know as i am considering a large appeture refractor project, although i was considering my first project with heavy duty PVC Conduit.