The Nikon BL-7 battery chamber cover for Z9 is now in stock everywhere in the US:
The BL-7 is a protective battery chamber cover is designed for use with the Nikon EN-EL18d rechargeable battery (or other EN-EL18 series Li-ion batteries) for the Nikon Z9 mirrorless cameras – it’s used to lock the battery in place in the battery chamber.