Hi Doug-Indycals has some resin tires for a select range of Indycar's-might work? Maybe worth a deeper dive on the manufacture's site for additional options?.
https://www.indycals.net/parts/indyparts/indyparts.html
I've had some success finding the tires from the AMT Indy Car series on eBay-but that can be hit or miss.
Good luck!
Bob