Where can I get a mystery box?

You can't just find that box in PoGo, you need to trade with a Pokémon Let's Go account to get it. If you don't have it and don't know anyone who does either, you'll be able to get a single Meltan from the Special Study I imagine - I haven't completed it myself, but like other Special Studies (Mew, etc.) it should provide you with one Pokémon to catch and a bunch of Candy. Since Meltan requires 400 Candy to evolve though, it's going to be a pain to evolve it without the Mystery Box.

With the release of Pokémon Go and Pokémon Home integration, you can receive a Mystery Box by transferring Pokémon into your Pokémon Home storage. This requires you to connect your Pokémon Go account with a Nintendo Account.

Linking Accounts

1. Click on the Pokéball icon and go to settings on the top-right of the screen
2. Scroll to the bottom and click "Pokémon Home"
3. Follow the on-screen sign-on options to sign into your Nintendo account

Transferring Pokémon

1. Click on the Pokéball icon and go to settings on the top-right of the screen
2. Scroll to the bottom and click "Pokémon Home"
3. Click "Send Pokemon", and then follow the on-screen steps to transfer your Pokémon into Pokémon Home.

After you transfer Pokémon over to Pokémon Home, you will receive a Mystery Box.