The first Leica M lens adapters for the new Nikon Z and Canon EOS R full-frame mirrorless cameras are already announced. The Shoten LM-NZ lens adapter (Leica M lenses to Nikon Z-mount) is already available for sale on eBay:
Shoten LM-CR lens adapter (Leica M lenses to Canon R-mount):
Novoflex Leica M and R lens adapters for Nikon Z mirrorless camera:
Shoten lens adapters are sold at eBay.
Novoflex adapters are available for sale at Adorama, B&H and Amazon.
Via NikonRumors