[xb-60]

Lippy - I am an ERA fan. If you have ordered an ERA car, 15" wheels/tyres are the only way to go (IMO). As has been stated if there is demand, there will be supply. There may be less choice, but you will be able to get them.
Again, IMO, don't go 17" or 18" unless it's for track use. These cars need to look period correct for road use, so they need 15" wheels and tyres.
My very best wishes with your ERA build.
Cheers,
Glen