Nikon hasn't made any extension tubes yet, there are some compatible extension tubes, I've found two brands which are all metal and with full contact pass-through:
Fotodiox (found by others in this thread), they have 15mm and 35mm, they come with a nice, expensive and useless wooden box, 111€ each (on Amazon)
Meike (nobody mentioned these yet), they have 11mm and 18mm, short lengths are more desirable than long ones, 35€ for the couple (on Ebay), other sellers offer them at double that (35€ each)
The Meike tubes are cheaper than a single Nikon Z bayonet, maybe a good starting point for experimenting in DIY adapters for exotic lenses. Bought one kit, it's under way, I'll report on the quality when they arrive.
Including photo.
Ciao fom Massimo