From what I know of, it is possible to get some of them, such as meowth and yamask. Some NPCs can trade you them. These NPCs are located in Pokemon gyms. I dont know if this is true for all, but some of them, definitely.
Edit: I just discovered you can also get Kantonian Mr. Mime in Spikemuth. Any other Pokemon that got a new evolution separate from an evolution that they previously had are available. Because Meowth and Yamask got Perserrker and Runerigus respectively, you can trade their Galarian Forms for their original forms because the original evolutions are actually required to complete the Pokedex. Pokemon such as Zigzagoon or Ponyta, however, you'll need Pokemon Home for. I don't know why you can get a Kantonian Mr. Mime because it only has Mime Jr. as it's pre-evo, and can't evolve into Mr Rime. I don't know of anymore possible regional variant trades in the games.
So this means that, unfortunately, you can't get Unovian Stunfisk in Sword and Shield yet.