I knew these were available since LOOOOONG TIME AGO, but I lost the track on them ( they were on ebay ). Making a search about some stuff this floated on suddenly, so posting the link for the future if you wanna get this pieces replaced on a mantenience job
https://premiumdashdecals.com/shop/all-car-lines-ammeter-insulator/
I have long attributed some ammeter connection problems to the original thin insulator shrinking with age. Better to replace it with a thicker material like micarta or phenolic; readily available in sheet form.