You can head to Gamestop in America, EB Games in Canada, and GAME stores in the U.K. to pick up a download code for Dialga in Pokemon Moon and Ultra Moon and Palkia in Pokemon Sun and Ultra Sun. The codes will only be available until February 28th.
You must redeem your codes in your games by May 28th.
To celebrate the distribution, the 10th movie, The Rise of Darkrai, is now available on Pokemon TV. It features Dialga, Palkia, and Darkrai.