First off, welcome to ChiefDelphi
There isn't a shortage of 8" pneumatic wheels. There is a shortage of AndyMark wheels. Huge difference. Also, AndyMark's are more like 7.5" at the crown with robot weight on them.
Seems like adapter plates from a subset of 8" wheels available on (e.g.) Amazon would be fast to manufacture at large scales, but at this point in the season it's tough to say.
Also, 6" wheels are still available from WCP. 8" wheels aren't the only solution to this year's game, and in some cases aren't the best solution.