Nikon USA authorized dealer B&H Photo Video and Adorama both said they will start shipping new Nikon D850 DSLR camera on September 7th, 2017. Amazon has updated the release date yet, but it is likely shipping in early September too.
From B&H Photo Video:
Expected availability: Thu Sep 7
Manufacturer will start shipping this item on 09/07/2017. Orders will be filled on a first come first serve basis.
In UK, WEX Photo also shows that D850 will be shipping on September 7th, 2017.