15" maybe more easily available than the 16" but I also found that numerous sites showed GS-D2 tyres and even allowed me to place an order but would then cancel, or worse, I would have to phone and chase up only to find they were going to ship me the GS-D3 instead. So it would be worth a call first to check stock.
I was at the MGOC Workshop this morning and was chatting to them about tyres. He said he's got a few 16's in rear fitment but said when he had those off the supplier they took a lot of getting, and he was told there definitely wouldn't be anymore. Looks like supply is really drying up then, not just the intermittent drought like normal.