Rob,
The Goodyear billboards are not that hard to find (just limited distributors). They look very good. However, they are bias-ply and non DOT (setting aside the issues associated with bias-ply tires, I believe they grip pretty well).
Avon makes a V-rated 15" tire in sizes appropriate for Cobras (limited feedback I've heard so far on these has been very positive).
Unfortunately, 15" performance tires for Cobras are pretty limited.
Check out
http://www.rogerkrausracing.com/Cobra/ for some of the different tires for Cobras.
Incidentally, it looks like you have a CSX 4000 car on order (aluminum, I'm guessing based on the CSX number). It sounds like you are considering a 17" rim? This moves you away from the "classic look" that you referenced (especially on an aluminum CSX car).