To celebrate the announcement of the new expansion pass and DLC for games, you can find a new NPC in the game that will give you the chance to catch a Galarian Slowpoke. You can also get a Shiny version of Slowpoke.
You will need to visit Wedgehurst Station. A new trainer has been added to the game and you can meet him there. The new trainer will be essential during the next DLCs, so consider this your introduction to future content.
A cutscene will play, and once it's done you'll be able to catch the Galarian Slowpoke. To get a Shiny, you need to keep resetting the match soft until one shows up. Now, there is a possibility that this meeting may be blocked, but at the moment we cannot confirm it either way.
If you're finally lucky enough to get a Shiny, grab it the usual way, weakening it a bit and then using a Poké Ball. If you don't want to reset the encounter indefinitely, you can get the normal Slowpoke, then use Hatching to try to get a Shiny.
You don't need to make any special purchases or pre-order the Expansion Pass to get Galar Slowpoke. All you need to do is visit Wedgehurst Station and talk to the new NPC you can find after the update.
The upcoming DLC for Sword and Shield will add over 200 Pokémon to the game, including many returning favorites. When the new content launches later this year, you can expect to spend a lot of time adding new Pokémon to your Pokédex.