Type: Null is one of those bizarre manmade Pokémon. It was created by the Aether Foundation in Sun & Moon. Then Gladion broke it out of the lab and made it his partner. So how is such a one-of-a-kind experiment in Sword & Shield's Galar region? Apparently someone stole Aether research and recreated the project. That means you can add it to your Gen VII team.
Where To Find Type: Null In Sword & Shield
As a lab-grown Pokémon, Type: Null isn't in the wild. Instead, you need to wait until after you beat the game and the Battle Tower opens for business. It'll be the tall tower where Chairman Rose used to reside. Go to the first floor, where you enter the elevator, and talk to a woman standing to the left. She'll give you a Type: Null.
How To Evolve Type: Null Into Silvally
Type: Null has its power locked thanks to that crazy helmet. To unlock it, you'll need to earn its trust. Play with it in camp, keep it at the front of your party, and don't let it faint. When it loves you enough, it will evolve upon levelling up.
What Silvally Does In Battle
The woman who gives you Type: Null will also give you Memories for each Pokémon type. Giving one of these Memories to Silvally will change it from a Normal-type to the type of the memory. It also has a powerful move called Multi-Attack that changes type based on its Memory. You can probably guess Silvally's role, then. It will fill whatever type you need on your team.