A rear bench seat can be bought from Audi, but it will be VERY expensive, as you cannot buy a complete seat, you;d have to buy the bases and the foam and the springs and the covers and so on.
Your best bet is to get one from a salvage yard although if you are trying to fit it into a car fitted with split rear seats, be warned, it is not possible to do, due to floor pan differences.
Cheers,
Mike