If someone can't visit a Gamestop store or you live outside the united states just follow the steps below:

1) Get an Android phone and install 'Fake GPS' from the store
2) Download the 'Pokémon Pass' from store or get it from apkmirror.com if it isn't available in your region
3) Get ready a Pokemon Trainer Club account
4) Run Fake GPS and search for a Gamestop store in united states, you can help you find one, using gamestop.com/stores/
5) Open 'Pokemon Pass' and Log In with your Trainer Club account
6) If you did everything correctly, you'll see the campaign promo on the right of your screen
7) You don't need a QR code, just touch the "?" button and touch 'Skip'
8) Touch to get up to 3 codes per account (you need more accounts if you have to farm more codes)
9) Use your codes on your 3ds. Shiny Solgaleo is only redeemable in Moon and Ultra Moon versions while Shiny Lunala is only redeemable in Sun and Ultra Sun versions.
10) Shiny Necrozma is a JPN only event so, just inject it lol

NOTES:

1) These codes only work on American consoles. Europeans must pick up their codes at other stores in their contries
2) Latest version of 'Pokemon Pass' is 1.0.4 if you plan to download it from an external site like apkmirror.com
3) A Gamestop store is currently in "900 E VICTORY DR, STE D-16, SAVANNAH, GA 31404" so you can go as quickly as possible on Fake GPS