Hi there
There are units for sale on eBay. Type in Audi A3 Cluster and something should pop up. Just make sure its the actual unit......there's a lot of ads for repairs/re-builds on your faulty unit.
If you get one.......you will have to take it to a garage.......best to take it to Audi and they will have to encode it for you.
The only difficulty would be.......On the Wiper stalk there is a button which you can scroll through....MPG, Miles until fuel, Average MPG etc.
I dont think you will have that so you would have to get one installed as well as the wiring and fuses etc.