In the past I bought whatever piece of rolling stock was cheapest, just to get this part. I don't have an immediate need for any more right now.
What I would rather have, is a retrofit collector roller kit for the most common MTH tender trucks. With only two rollers on the loco, MTH steam locos are prone to stalling on back-to-back switches and crossings, especially if they were ever used on RealTrax with the blackened rail.
There is already a humongous tether between loco & tender carrying track power to the PS2 board. Putting rollers on the tender would yield a clear performance improvement, except that there's no obvious way to do it! There's not even a good reference document showing the various types of MTH rollers.
As with Lionel, MTH tenders have different types of trucks but if you're planning to 3-D print these things, low production volumes shouldn't be a big deal. There's your market opportunity IMO. -Ted