A quick update: the Nikon MB-D18 battery grip for the D850 DSLR camera is currently in stock at Adorama, Pictureline and Huppin’s (all are authorized Nikon US dealers).
Update: the $940 battery kit that gives you 9fps for the D850 is also now in stock at Adorama.
Here is some additional info on the new Nikon EN-EL18b battery and its performance: