From October 22nd, 2021 until November 11th, 2021, players of Pokémon Sword and Shield can receive a free distribution code in-store at participating GameStop locations to unlock a Shiny Zacian for your Pokémon Shield game and Shiny Zamazenta for your Pokémon Sword game.

Country Distribution List:

US: Gamestop - Shiny Zacian - October 22 – November 11, 2021
US: Gamestop - Shiny Zamazenta - November 12–26, 2021
Canada: Gamestop - Shiny Zacian - October 22 – November 4, 2021
Canada: Gamestop - Shiny Zamazenta - November 5–18, 2021
UK: GAME (In-Store and online)
Germany, Austria, Switzerland: Gamestop (In-Store only)
Ireland: Gamestop Shiny Zacian and Shiny Zamazenta (In-store & Online)
Spain: GAME (In-Store & Online), FNAC (In-Store)
France: Micromania (In-Store & Online)
Italy: Gamestop (Online)
Belgium, Netherlands: GameMania (In-Store & Online)
Denmark: Nintendopusheren (In-Store), NØRDCON (October 23rd/24th), Proshop (In-Store & Online)
Sweden, Norway, Finland: Proshop (Online), Spel & Sånt
South Africa: Nintendo Distributor Email
Japan: Participating video game retailers nationwide in Japan
Malaysia, Phillipines, Thailand: Various gaming stores (In-store)
Singapore: Pokémon Center, Best Denki*, BS SONS, Card Nation, Challenger*, Courts*, CynicsSG, Eplay, Family-Com, G & E Gaming, Gain City*, Game Academia, Game Resort, Game State Singapore, GameMartz, Gamewerks, Gamextreme, Generation Games, Happyland Collectibles, Harvey Norman*, Parisilk*, PLAYe, Pokémon Center Singapore, Popular*, Qisahn.sg, RPG - Right Place for Gadgets, Starled, Tangs*, Toy or Game, Toys Terminal, War Games, Zark Games
Hong Kong: 1O1O center & csl (In-Store)
Taiwan: 7-11

Get Shiny Zacian and Shiny Zamazenta at GameStop

Visit the retailer to receive a code card that lets you bring these Legendary Pokémon into your Pokémon Sword or Pokémon Shield game.

Are you a Pokémon Sword player who has been itching to add Zamazenta to your game? Or maybe you're a Pokémon Shield player whose Pokédex has a noticeable lack of Zacian. Well, you're in luck! Soon, you'll be able to get Shiny Zacian and Shiny Zamazenta by visiting a GameStop store and picking up a special code card.

The codes let you receive Shiny Zacian in Pokémon Shield and Shiny Zamazenta in Pokémon Sword. Both Pokémon arrive at Lv. 100, and both Pokémon come with their trademark held item—a Rusted Sword for Zacian and a Rusted Shield for Zamazenta. These items allow the Pokémon to change into their powerful Crowned Sword and Crowned Shield forms, respectively.

Check out the schedule below to see when you can get code cards from your local GameStop store.

• Shiny Zacian
  • October 22 – November 11, 2021 (US)
  • October 22 – November 4, 2021 (Canada)
• Shiny Zamazenta
  • November 12–26, 2021 (US)
  • November 5–18, 2021 (Canada)

To claim your in-game gift, follow these steps:

1. Launch your Pokémon Sword or Pokémon Shield game.
2. Select Mystery Gift on the X menu.
3. Select Get a Mystery Gift.
4. Select Get with Code/Password to save your game and connect to the internet.
5. Enter your code or password.
6. Watch as the gift arrives in your game. (Pokémon will appear in your party or your Pokémon Boxes. Items will appear in your Bag or in the fitting room of any boutique in the Galar region.)
7. Be sure to save your game.

There's no purchase required for this promotion, but the offer is only available while supplies last, so don't delay. Head over to GameStop to claim your Shiny Zacian and Shiny Zamazenta!

Source: https://www.pokemon.com/us/pokemon-news/get-shiny-zacian-and-shiny-zamazenta-at-gamestop/

GameStop Distribution Code

Receive Shiny Zacian for your Pokémon Shield game and Shiny Zamazenta for your Pokémon Sword game. Get a free distribution code in-store while supplies last! Check your local GameStop store for details.

Sources: https://www.facebook.com/GameStop/photos/a.126580857361849/4840519925967895/ and https://twitter.com/GameStopCanada/status/1450102456319463428

Singapore Distribution Code

During October 22 (Fri) to November 18 (Thu), special Serial Codes where you can receive Shiny Zamazenta in Pokémon Sword and Shiny Zacian in Pokémon Shield on Nintendo Switch will be distributed at the designated game stores in the list below. A decent amount has been prepared, but the distribution will end once the codes have ran out.

How to receive the Serial Code

Please tell a staff at the store that you want to receive Shiny Zacian and Shiny Zamazenta flyer

Regarding the Serial Code

You will receive a flyer that has two Serial Codes;
a Serial Code to receive Shiny Zacian in Pokémon Shield
a Serial Code to receive Shiny Zamazenta in Pokémon Sword.

*The flyer is limited to one per person.
*The Serial Code can only be used once per save data. It cannot be used twice or more.

Pokémon

- Shiny Zamazenta for Pokémon Sword
　 Held item: Rusted Shield
- Shiny Zacian for Pokémon Shield
　 Held item: Rusted Sword

Distribution Period: October 22 (Fri) to November 18, 2021 (Thurs)
Code Redemption End Date: November 25, 2021 (Thu)
Store List: Best Denki*, BS SONS, Card Nation, Challenger*, Courts*, CynicsSG, Eplay, Family-Com, G & E Gaming, Gain City*, Game Academia, Game Resort, Game State Singapore, GameMartz, Gamewerks, Gamextreme, Generation Games, Happyland Collectibles, Harvey Norman*, Parisilk*, PLAYe, Pokémon Center Singapore, Popular*, Qisahn.sg, RPG - Right Place for Gadgets, Starled, Tangs*, Toy or Game, Toys Terminal, War Games, Zark Games
* Note The Pamhlet will be distributed in only selected stores where video games are sold

Source: https://sg.portal-pokemon.com/topics/game/211014130000_receive_shiny_zacian_and_zamazenta.html