Hmm, I'm assuming you've tried eBay, Gumtree etc. My other suggestion would be to see if Audi still stock them. You'd be surprised, as wheel centres are easily lost :)
I think what would help us (as in you and me) if we knew what the correct name for these wheels are. If we knew that then it would help us find new center caps. If you see what I mean.
Cheers
Steve