At Tyres.co.uk, you can buy Continental and Firestone tyres in the size 175/60 R15 81V for your Audi A2's 15-inch factory wheels. Since other models of the A2 might have different factory wheels, make sure you consider the size of your wheels and check the Load Index and Speed Rating in your registration. The size of your wheels is stated on the placard.
The A2 has its factory information printed on the sidewall of the tyre. The factory placard has the approved combinations of wheels and tyres with the corresponding monthly air pressure. It also contains the information on the B-pillar in the door frame of the driver's side. For a tyres and wheels replacement, use your handbook, as it contains information that can help you verify the fit.
Since the A2 does not have quattro, you do not need all four tyres to be in a matching set. Replace tyres as a matching pair on the same axle. When fitting two new tyres to a set, install them on the rear with matching size, construction, load rating, and speed rating.