Durrell Armstrong at Player Piano Co., Inc., still carries "Gummed End
Tabs" complete with D-rings in his current (1994-95) catalog, at $10
per 100, but the D-rings are not listed separately.
It would be easy to fabricate D-rings, out of 2 mm round brass wire
stock, by making a bending jig out of 11 mm round steel and cutting a
transverse slit about 3 mm deep in one end, which is then held firmly
in a bench vice.  The brass wire is simply bent round the jig and cut
off as required.

That having been said, the end product will probably not be as
professionally perfect looking as Durrell Armstrong's.

David Snelling
Isle of Man