I received several tips from readers that a third Nikon D850 shipment is now reaching US retailers (see this and this blog comment and several reports from the Nikon D850 Facebook group that now has almost 10,000 members).
The first D850 shipment was in the first week of September, the second shipment in the US was around September 18th. It seems that Nikon USA is shipping the D850 camera to retailers every two weeks. The next (fourth) shipment is expected around October 10-15th.
Amazon has the Nikon MB-D18 battery grip listed to start shipping in 2-4 weeks.