Both Numrich/Gun Parts and Brownell's have cataloged generic magazine tubes, cut them to length and file and drill whatever features the gun requires. Check the sites for current offerings. Also mike your current tube diameter to see which size you'd need.
The only thing holding a Marlin magazine tube longitudinally is the barrel band and screw, the screw should be fitted so half is in a groove in a barrel, half is in a groove in the magazine tube, both being tight fits. Sounds like your gunsmith didn't do that. The magazine cap screw has a tit that extends into a groove in the barrel. So if you want a factory-type fitment, you'll need some milling done for the groove(or have a VERY steady hand with a Dremel and cutoff wheel). I have also seen a dovetailed piece that an extended cap screw fits into, the cap is just clearance drilled, not threaded, and there's a raised portion around the screw hole in the dovetail piece that fits into a counterbore in the magazine and magazine cap. I saw this on a custom lever that had heavy recoil, one of the Alaskan guns. With a Marlin, you do NOT want a fixed magazine tube connection at the receiver, think of what will happen to your point of impact if the barrel is fixed at the ends and it heats up.
You're still going to need a barrel band, however you do it, and will probably end up with an unused notch in the barrel. You could chop it back past the notch if that offends and get it redone properly, more money in the project, though.
Stan S.