Try Wheel Vintiques. WV has custom and repro wheels for just about anything and have a full line of trim rings and caps, including baby moons.
Coker Tire lists Wheel Vintiques now in their catalog.
You might find them at a local speed shop too. My experience has been to avoid the chromed ones available these days as the plating is a one step disaster that pits and rusts real quick. Stainless caps are the way to go.
