This is for the owner of the RS/Z above who wants some flat caps for his tastefully modified Z.
Here's a page of center cap providers, hopefully the contact info is still current. It's on Vintage Wheel Works' website, but their wheels are built with an essentially similar center pattern as American Racing Torq Thrust D's, and most of the centers are marketed to Torq Thrust wheel owners.
http://www.vintagewheelworks.com/centercaps.html