The answers I'm finding/getting are that the G8 Pontiac center caps (pn: 92203234) are the largest Pontiac center caps out there.
If you know the size in millimeters that you need, you could try finding some universal/plain caps on ebay and perhaps use an arrowhead overlay or decal.
Good luck! I hope I'm wrong and you find actual Pontiac caps!
Let us know what you end up going with...