Available again – the Nikon Z9 camera is currently available for order at Amazon with a shipping date of August 23 September 1 (shipped and sold by Amazon):
It seems that Amazon is releasing small batches of the Z9 on a regular basis – keep checking the Z9 listing.
B&H Photo is currently shipping Nikon Z9 pre-orders placed in February 2022.
Adorama expects the Z9 to be in stock on 01/03/2023.