Hi,
You can obtain a replacement SD card from an Audi dealer, who no doubt will charge not only for the SD card, but for activation as well, and that won't be cheap! You can obtain a card from a number of the Navigation updates sellers on the internet, but from feedback on some of these sites, they don't always work.
If the car was advertised with Sat Nav then it's up to the dealer you purchased the car from to provide the SD card, or get them to pay for a replacement SD card and activation by an Audi dealer.
Good luck.