I think they were originally for Ferraris.
Anyway, best to give Dougal a call at Longstone tyres and see what he says, might well have a US outlet. You will find it hard to find anyone more obsessed or knowledgeable regarding tyres.
One other option is these cheapies which have just become available in the UK (we are gettng spolit for 15" choices now!).....dont know anyone thats tried them, but they are V-rated.
255/60R15 Galaxy R1 GT Rasied white letters
- assume these are available on the US. Noone I know of has tried them on a cobra yet.