I'm the author of the above post - I know all about rear diffs! Mine is a late 2014 build (car delivered early 15) and I had the F variant on my car. You can get any 0CQ525010X where X is what you have or later; there were D, E, F, H, K, J, L and S I believe (S is the current version you get new from Audi if you fork out £3,800!!!)
There are a few knocking about on eBay, what I would say is play the waiting game, not many of these sell, if you watch the items or message sellers they'll give you big discounts on the prices they ask on their listing. If you look at the thread above, I found one guy that was willing to sell for around £200. I'd be weary of West Yorkshire-based dealers, the one I got was from a Nottingham breaker and (touch wood) all fine so far