I don't think the frame parts are stamped njs in fact. They are probably approved for njs frame like tubing can but at the end, the njs stamp is suppose to be on the bottom bracket shells not on the frame part. So it should be possible to buy them.

But you right, it should be easy to make.

I tryed to contact track super market to see if it's possible to find somes.
Marinoni in Canada have nice dropout too and not really expensive. I saw them on a Phil Barge frame,...really nice.