Pirelli has (or had) a 'Special Projects' tire building shop in Izmir (eastern Turkey) where limited market tires have been built for decades. Problem is, I understand that's also the tire shop where all the F-1 and other pro racing tires are built. Guess which takes precedence during the racing season? Nevertheless, a few such tires do trickle out- to be instantly grabbed by Euro dealers who have standing (paid-for) orders.
That's what happened up to about 2015, anyway. You need to talk to someone in the know located in Europe about today's situation. The Swedish Pantera Owners Club has gotten hold of a few dozen tires that way in the past, but the market is limited for 285-50x15 for 10" Campy rears and a few other brands of street cars. While the smaller OD 285-40 x 15s are used for Countache, Porsche & wide-body Pantera fronts; much bigger market. My info is dated- the last tires I got thru the Swedish Club was early '90s and that was a group buy with the European Lambo Owners Club. Good luck-