Same as the last few years: Walther makes a batch, sends it over, and it sells. Then a few months/years of drought and you'll have to wait until Walther sends over the next batch. So that's where you are: wait! This year's batch came over in early January, and was snapped all up and every place that had them is out of stock.
OP - I am seeing a couple of used full size 9mm P99 AS pistols listed on GB right now if that's the variant you're looking for. The starting bids are a bit high but not unreasonable. The .40S&W versions seem to always be around during shortages. The 9mm versions have a tendency to disappear rather quickly. I remember earlier this year when this forum was buzzing with the news that a new batch had come in. People were snagging them up at incredible prices... way less than I paid for mine back in 2016. Be patient and I am sure you will get one!