Should have been paying attention at Le Mans Classic! Plenty of cars on 15" rims there and plenty running Avon CR6Z's - sticky rubber but period look.
I believe Hawk use them.

BF Goodrich Radial TA's are still relatively easy to get - try North Hants Tyres - but low speed rating.

Comp T/A's on the other hand - now that's a different matter

I made a list some time ago of 15" wheels here: http://www.btinternet.com/~scott.akister/tyres.htm