As D. A. Karp wrote, you can make decals yourself using
ALPS MD series printer. I heard that there is dedicated
mailing list on this topic. I use MD-1300 (discontinued),
but I don't join that list.
Note that available color is limited. It can prints "solid"
CMY (cyan, magenta, yellow) and their overprinting combinations.
I think its solid cyan is good for the Amtrak's unit number decal.
Any other tone is represented by dithering. In my opinion,
it is not acceptable quality.
Also note that its resolutions higher than 600dpi are for
special photo-paper printing, not for decal. (but I don't know
latest machine spec.) So you don't need to purchase higher rank