Barry,
As you know these are now quite rare.
They were not painted black on the original cars and they were often retrofitted.
The original Dzus fasteners were supplied to FAV from the local aircraft parts suppliers in Slough and the Southco UK branch still exists. I have the old 1966 UK catalogue and purchased some samples from them but the full Dzus stamp is missing so we resorted to searching the web for NOS parts and we got lucky.
Good luck with your search.
All the best,