Are you speaking of the receiver? If so, you won’t find a source. I believe they had either stamped out or milled the pattern from 2mm sheet metal, and then rolled the metal into a cylinder. After that they spot welded everything together down the seam. If you look closely you can see the holes of the barrel are not perfect circles, because they were added before the metal was rolled.
I know they make welded steel tubing for hydraulic applications, but trying to find something to mimic the MGC receiver tube would be really difficult. I would suppose if you were hell bent on going as accurate as possible, you should reach out to some laser cutters in your area; it may be feasible to source the solid tubing and laser cut the tube length. Just thinking out loud.