The cheapest tyre site is Camkill, then get them fitted at a 'wheel safe' location. Expect to pay around £170 for the 255 tyre.
My car had spacers on until 10k miles when those tyres were fitted, I ran no spacers for the second set and they wore the same. My car isn't lowered, unfortunately... only because Audi told me my warranty would be void as far back as the gearbox. Talking of warranty void, I've got a rough running idle at the moment where the car is spluttering and after 2 visits Audi have refused to pursue it any further having all of a sudden discovered I have a 'non-standard exhaust', they are only interested if I install a replacement standard exhaust. Sick and tired of this easy opt out warranty!! So I am paying for the spark plugs to be replaced tomorrow, if that doesn't fix it I will pay my independent to solve the problem, somebody with skills beyond an ECU scan. Lots of people with a standard set up with this issue, as well as modified. I know of a guy who fitted RO2 tyres to his PFL RS3, blowout within a few months (undefined mileage) due to heavy inside edge wear, that car was lowered and heavily spaced however.
13k miles is more reasonable providing they are safe, but
@FBF01 's tyres have been on the road to catastrophic failure for some time looking at that.