First of all the title is misleading and it should read something relative to what you're looking to buy, not what you got already as that's too abroad and totally irrelevant!
Secondly what you want is less likely somebody sells as nobody would sell the bottom section of an S3 rear bumper, if they do then something must have happened to the bumper itself
Best place to look on local scrapyards, eBay, gum tree and if I could remember right there was a website which it was selling performance cars parts, I may have it bookmarked or something, If I found it then I'll let you know
In regards to parts number, I guess I had that somewhere too, but first thing is first you need to give more details about your car, Is it sportback or hatchback etc...
Cheers
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