As far as I know there is no source for them. Possibly someone has some old stock or used ones off a parts motor. They are not available new. I did see a guy on FB make his own from Delrin, but he wasn’t selling them.
Try a wanted ad on the classified section.
The phenolic compression fittings are often broken during removal. You have to be extremely careful. I find a heat gun helps. Anyway, my point is that you will probably have to buy an entire mag plate to get those fittings.
A couple of long-shot alternatives:
– Carve phenolic board with a router and/or a milling maching. Metal tools really work well on the board.
– Find someone with a 3-d printer. The printers can produce machinable products.