I'm in the same boat, Pro Race 1's are in the mail and I need to start thinking about the center caps. Peter at Rspeed said that one of their customers found 60mm caps at a local tire shop, maybe an NTB, he was going to try and find the details. Are your Pro Race 1's relatively new, and you've confirmed that they are 60mm? I read something subtle about a change in the center cap design recently.
In the meantime, Lowe's looks like my best bet, we do have them in NC. I have not yet looked but probably will this weekend. Searching the web, I found several electrical supply sites which carried Carlon. Here's one which actually shows this part:
http://www.munroelectric.com/catalog/carlon/bodies.html
It almost looked like they wanted to sell by the carton, though. Wanna make a carton of these? I'll buy 4. ;-) I'm not sure I've got the tools to do a nice job on this.
I'll let you know if I find them at our Lowes or I may be placing an order.
Later,
Scott