Well, we found a set in the USA simply by looking for the last person who had them!
You can purchase the sets as well from aftermarket suppliers, in the USA, although, as usually happens with overseas members is the taxes and shipping kind of prevent them from doing what they would like.
In less time than it takes to order and wait for them to arrive, the body shop I used some years ago had 10 hubcaps all masked off by hand and painted. Former member Tom Rose and I both have 568 signal red cars thus we did them together.