Pokemon Sword and Shield are continuing the theme of recent Pokemon games, including many regional variants or reproductions of previous generations of Pokemon. Yamask is one of the variant Pokemon in the Galar region, which was introduced in Black and White originally. If you don't have one, you can catch it in the game. The PKMBuy is the best store to buy Gigantamax Pokemon. Here is the best way to capturing a Galarian Yamask.

Galarian Yamask
Galarian Yamask can be found in Pokemon Sword and Pokemon Shield on Highway 6, between Hammerlocke and Stow-on-Side. They often spawn in the overworld, so it should be easy to find a roaming on the edge of the cliff. They range from 29-33 levels in the wild. Throwing at a Quick Ball should be enough to catch it, but if it needs to be weakened it should have enough defense to withstand one or two physical attacks without being Knocked down. 
It is a Ground/Ghost-type Pokemon, making it completely immune to Fighting, Normal and Electric-type moves. In addition, its ability is the wandering spirit, and when hit by a touch action, it exchanges with the attacker's ability. This makes it similar to Cofagrigus' Mummy and Unovan Yamask, although this ability will only spread to others. Wandering Spirit does not affect certain skills, such as Silvally's RKS System or Mimikyu's Disguise. 

If possible, it is best to capture Galarian Yamask which has a Relaxed nature, which will increase its defenses and reduce its speed. Also, many new players Buy Gigantamax Pokemon instead of catching one. If not, Impish nature may also work, which will increase defense and reduce special attacks. There is no rush to evolve Yamask; it learns all the same moves as its evolved form—even learns faster. Whenever you choose to evolve Yamask, just make sure that its total HP is at least 50.