Shimano MF-Z012 5 Speed Freewheel Shims
I don't know of a source which sells these parts new, but...

As a volunteer at the local bicycle co-op we literally have a bins full of old freewheels and sell them for a few $$$.  If you have a bicycle non-profit near you, I would be surprised if you cannot find a couple similar freewheels which you could use for donor parts.

There are Facebook groups which cater to vintage bicycles and many folks in those groups either sell vintage parts or know others who might have a source.  Here are a couple:

The Vintage Mountain Bike FB Group

Vintage Cream FB Group

You could also check with some of the online groups.  The Velocipede Salon has A LOT of members who are (were?) long time bike builders, mechanics and shop owners.

Velociped Salon Group

Good luck with your search and feel free to contact me directly and I can see if the Co-Op has a similar freewheel.