Look at it this way, you are looking for a part for a gun that hasn't been sold for years.
The part in question isn't normally one that 'goes bad'. Most lever actions never need a new mag tube through the course of two or three owners and 50 or 70 years.
The part being for a .454 we suspect it is unique to that caliber as we know Rossi didn't thread any of the more common calibers. At best the same part MIGHT have been used for the 480 Ruger chambering.
So you are talking about a replacement part that doesn't normally need to be replaced for a what amounts to a very limited production that ceased some time ago. I'm guessing it's doubtful that Interarms EVER stocked replacement mag tubes, and if they did so it would have been either the inner tube assemblies OR for models they imported much larger quantities than the .454. While Rossi no doubt stocked the parts during manufacturing they may never have considered them 'spares' and given their parts policy may well have considered them a part to only be replaced by 'manufacturer's authorized agents'. EVEN if either of them carried spares once they were done selling them, and after another six months or a couple years, they probably sold any remainders to the parts pirates to clear out the space.
So you are more or less stuck either doing as good a refresh on the existing tube as you can OR having a replacement tube fabricated. To do that you have to find a tubing that is a close match to the OD so you can make it fit the receiver and barrel bands. It has to be a close match to the inner mag tube so that fits. Then you have to cut threads to match the receiver threads. That can be a bit of a pain as it isn't uncommon for threads to be a bit of an odd pattern. Even if they aren't square threads or some other oddball speciality form they can be a pain to match the existing threads and get the die to cut them. Cutting the other features in the tube wouldn't be very hard.
The only other likely option would be to find a donor tube from a gun that was decommissioned but stripped for parts. There are several parts suppliers that offer parts from that sort of thing but what they have and what they charge varies a lot and once again you are stuck trying to find a part from what is a pretty rare gun making the chances of finding a parts pirate with it somewhat unlikely.