Can't you just go loose ball? If it takes 3/16" balls, buy about 50 (probably use 44 or so), if it takes 5/32", buy about 60 (expect to use 50ish), and get the whole Campy Pista HS experience.
Failing that, your LBS should have several choices. Hatta, Tange, and Shimano seem to have used maybe five different retainers tops over the last 30 years. Bring in your frame and your top cup (clean, please) and try a few until you find a fit.