M4A1 Custom Gold

Other than crates and events you can obtain it temporary here:

ZM2 Battleship on Medium you can get it temporary (7 days)
ZM2 Shipyard on Medium you can get it temporary (7 days)
ZM2 Shattered Station on Medium and Hard you can get it temporary (7 days)
ZM1 Final Arena Special Box you can get it temporary (7 days)
ZM1 Elemental Temple S clear reward you can get it temporary (7 days)