I bought my 8 from a dealership with 10k miles on it. The keychain just had the remote and the normal key on it, not the valet key.
I haven't run into a situation yet where I'd need it, but if I did, how would I get one?

I know this guess is gonna seem rather obvious, but I'd imagine it the only place that would even be able to help you out at all would be the dealer. Rather it be that they try to order/cut one, or replace yur ignition. Hopefully not the latter...

Same thing happened to me. See http://www.rx8club.com/showthread.php?t=78392&page=2&pp=15& -- first the dealer tried to screw me but then the story had a a happy ending. Sequel: six months later they found my original factory valet key, the one they said didn't exist. Of course since by then they had programmed a new one, the original valet key wouldn't start the car anymore, although it does still open the doors.