Tank bag since march myself, however I've just been to Austria and them and the Germans and Italians i met on the stelvio pass have fully loaded bikes with every extra available without issues.
I stayed in a hotel in Tyrol region and they hire'd BMW bikes and gs Lc came with vario's tank bags, i asked the owner if they had issues with getting the extras no such thing.
It's very odd that people are still waiting, as I had everything through by mid April (varios, tank bag, engine bars, Akra, tinted screen, sport seat and footpegs).
I ordered my TE with large tank bag. When i collected the bike it wasn't available so a few day later i was passing Rainbow BMV and asked when they could get me one and the guy said no problem we have them in stock and that was early April.
If I was you I would have a ring round the network and I'm pretty sure you'll get one. Good luck.
I had to sleep with grown men (again) to get my hands on a set of panniers.
But seriously, it seems a mixed bag, there doesn't seem to be any consistency on supply - being positive and always giving the benefit of the doubt lets hope its because the bike is soooooo popular!
My new F800 Adv is a month late but that's fine as the dealership is Rainbow and they seem really good guys with a good reputation.