UPDATE] Motorola may introduce a Moto Mod with a second display - KLGadgetGuy
Amazon.com: 5G Moto Mod for Motorola Z4, Z3, Z2 Force - Ultra Fast Internet, Built-in Battery, Mobile Hot Spot - Black - Verizon Network : Cell Phones & Accessories
Polaroid's new $200 Moto Mod turns your phone into an instant camera | Mashable
Here's every Moto Mod you can buy for the Moto Z series | Android Central