A warm welcome to the forum and your optimistic post sees you joining the happy throng of owners who'd like a leather interior for our beloved A2's!
The odd interior comes up at a breakers, and I have seen a couple on eBay - (now been searching for almost a year) You also have to ensure the donor car has the correct rear seat configuration ( 50-50 split or full bench seat)
To be realisitic, if you really desire this option, the best bet is a retrim at a reputable vehicle upholsterer. This starts at about £800 ish.