The Pokemon Company has announced that both Heatran (Ultra Moon and Moon) and Regigigas (Ultra Sun and Sun) will be distributed in North America and Europe from March 1, 2018.
In North America, Pokemon Trainer Club members will receive a code to redeem either Heatran and Regigigas (depending on the version of the game), as long as they sign up by March 1. The codes will be sent via the Pokemon Trainer Club newsletter. Sign up here.
In Europe, both Pokemon will be distributed via Nintendo Network through the Mystery Gift function. The Pokemon they will receive depends on the version of the game.
Check out the full list of distributions for 2018.