I'm pretty sure you're limited to a Q5 wheel & only a Q5 wheel. I managed to get one from ebay after a very kind fellow forum member posted it up here. They come up from time to time, so keep an eye out. Alternatively, pop into your local Audi dealer & sweet talk them into ordering a spare with the next demo Q5 they get in so you can buy it from them!
Theres some Q5 alloys on ebay at the moment but no collapsible spares. They're like rocking horse doo doo I'm afraid and fetch a premium when they do come on. Get on at your local dealer as Dogzy says.