Yamaha don't make a specific tail bag.
Ironically I didn't even think about a tank bag, even though I've had a bagster tank cover with mounts on for the last year. I've just kept it on to stop the tank getting scratched when parked amongst the bikes outside work.
I'll take a look but I don't think they do any tank bags which still give easy access to the filler cap.
Had a look at Kriega but they look like bags and I'm quite vain, I'd like something which looks like a small shell bag, similar to the hard shell type motorbike backpacks.