I've never seen anyone selling magazine housings or tubes. I thought one of the surplus parts sellers like SARCO or Gun Parts Inc might have but, at lest at the present time, they do not have anything.
Depending on where the crack is, a fellow good with a TIG welder might be able to repair the magazine housing. Many moons ago before high capacity 1911's magazines were available on the commercial market, I took two GI 1911 magazines and spliced them together to make a higher capacity magazine. I think it was a 9 or 10 magazine. It worked.
I suspect that the housing is the most expensive part of the magazine. The spring and follower are wear items anyway and not very expensive to replace. So, if the housing gets damaged, it probably is about as cost effect to replace the entire magazine as fooling with rebuilding one.
Maybe try contacting one or two of the magazine manufacturers that sell magazines directly to the consumer. Maybe if contacted directly, they may sell you the housing/tube.
Personally, I do not think it is worth the effort. Just buy a replacement magazine. Save the spring and follower for spares.
