This battery mod for the Moto Z, Moto Z Force, and Moto Z Play handsets appears to be a Verizon Wireless-exclusive product and carries a $79.99 USD price tag. Fortunately, this accessory should work fine with unlocked Moto Z phones as well, but also with those on other networks.
http://www.notebookcheck.net/Mophie-Moto-Mod-battery-pack-for-Moto-Z-hits-Verizon.184651.0.html