Starting on December 8th, 2019, Pokémon Center and Pokémon Stores nationwide in Japan will be distributing a Completely Understanding the New Pocket Monsters Anime Book (新アニメ「ポケットモンスター」まるわかりブック) at the checkouts. The book comes with a serial code for 100 Monster Balls which can be used with Pocket Monsters Sword or Shield.

The book features the Pokénchi members thoroughly introducing the charm of the new anime series! Get a complete picture of the adventure, including info on the characters and Pokémon from the new series!

Source: https://www.pokemon.co.jp/info/2019/12/191206_p01.html