You might want to check with a tire store that sells a variety of custom wheels. When I purchased the new wheels for my '75 Cadillac tow vehicle, my local Plaza Tire Store had centers the same shape as the one on your wheel in a variety of diameters and depths. Their prices ranged from about $12 to $20 each. I can't remember what the manufacturer's brand may have been - - I know the wheels that I purchased for the Cadillac are Boyd's - - they carried at least four different brands as it took a number of days to get answers from each of the vendors on wheels that were available to fit the early front wheel drive Eldorado.
Good luck with your search!
Kevin D. Allen WBCCI (Lifetime Member)/VAC/Free Wheelers #6359 AIR #827
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)