After you become Champion, you can find Gladion and get a Type: Null from him.
How to Get Type: Null
Go to Aether Paradise and take the elevator up to 2F. Go to the north middle end to find Gladion. Talk to him to get Type: Null and all the memory drives that Type: Null's evolution, Silvally, can use.
How to Evolve Type: Null
Type: Null will evolve into Silvally when leveled up with high friendship/happiness. You can increase friendship in the following ways:
- Give Type: Null a Soothe Bell to hold. It will double the amount of friendship that the pokémon earns when you do the following things.
- Put Type: Null into your party and walk around. Your pokémon's friendship will increase every 128 steps you walk. You can ride Tauros to do this more quickly.
- Type: Null's friendship will increase every time it levels up.
- Type: Null's friendship will increase if you use Vitamins on it (e.g. Carbos, Iron, etc.)
- Take Type: Null to Konikoni City on Akala Island and talk to the hula dancer to give your pokémon a lomi lomi massage, which increases friendship. The hula dancer can only give one massage per day.
- Put Type: Null into your PC Box (which will NOT change its friendship), then go to Poké Pelago and put Type: Null into the hot spring in Isle Avue. Your pokemon's friendship will gradually increase over time while in the hot spring, but be sure not to leave your pokémon there for more than 24 hours or its friendship will start to decrease. You can add Poké Beans to the container next to the hot springs to make your pokémon's friendship increase even more quickly.
- If you have a Friendship Café in your Festival Plaza, you can purchase meals for Type: Null to increase its friendship.
You can check Type: Null's friendship by talking to the female veteran in Konikoni City on Akala Island. If she says "My! It feels incredibly close to you! Nothing makes it happier than being with you!" then Type: Null will evolve into Silvally the next time it levels up. If she says "You clearly love your Pokémon, and you must spend a lot of time together." there is a chance that Type: Null's friendship is high enough for it to evolve, but you might want to increase its friendship more, just to be sure.