In the US I got my XT linkglide Di2 derailleur, XT shifter and CN-LG500 chain from PerformanceBike and the cassette from a random bike shop that happened to have it on sale.
Thanks but I got what I needed from an industry contact in the end. I got the XT derailleur, 11/50 Cassette (which is pretty stoutly built) and as long as I was in with the hookup I got three chains. Didn't need the shifter but the inside guy was pretty sure I would in the end.
Came the other day and got it set up on my bike. Had to take a link out of the chain and do a bit of fiddling with the b tension screw and adjusters then I tuned my Archer Components shift system to the cassette for a working man's e shift and life is now good and shifts better than it ever has from new. For how long? Who knows? More test time needed, which is the good part.
Since then I saw on another thread that there is a grouppo on eBay available. I did the usual Google search and found nothing which was strange. Almost as strange as how it is in stock at Shimano and not in the usual parts haunts?