Audi is popular in the UK, but higher end models like A6, A7 A8 etc is less so and more difficult to find (compare post count on the A3 forum to the A6) and when it's a limited run like the facelift model the parts are even harder to find, and like yourself people what to upgrade their pre facelift model to a FL version so the parts are in high demand.
To be fair when I'm looking for parts I look on ebay.de but it's best you have the part number unless your German is good.