Where can I get one of these???

The problem with making one from a nylon block in a milling machine is in those square inside corners.
If a small diameter mill is used you can grind or file matching radius on the corners of the mating metal part(s). Otherwise you would need to make the nylon block larger to accommodate a relief radius beyond the metals corners.
I suppose you could try to make one from a product like Plast-Aid but.. Anything that cures as fast as that stuff does is going to curl/draw/shrink, more so depending on the size and thickness of the part. I've got some 2 part acrylic that will work but it takes a couple days to fully cure.