The shiny variations of Pokémon in Pokémon Go are incredibly difficult. It’s a little easier if you’re hunting the ones you’ll find during raids, if you’re willing to waste a raid ticket for the event. If you really want one of the rarest shiny Pokémon, you will hunt for shadow shiny Pokémon. The shadow legendary Pokémon is rare and comes only from Giovanni, but the shiny shadow is even rarer.
There is only one method to get a shiny shadow Pokémon. You can do this by fighting one of the three leaders of Team Rocket and no, Giovanni is not involved. You must fight Arl, Sierra and Cliff. These three will rotate their first Pokémon every few months, and this new Pokémon will be a new potential shiny Pokémon that you can capture. During the battle, you will not know that they are shiny Pokémon, so you have to fight them, win and then try your luck when you find out if the shiny will arrive when you try to save it.
None of the Team Rocket grunks you’ll meet at Pokéstops can have a shining shadow Pokémon. You can capture their shadow Pokémon as a reward, no problem, but you don’t have a chance to find their shiny version.
Pokémon, Arlo, Sierra, and Cliff, take turns so often, so you may need to investigate a bit to see what shadow Pokémon you can capture. The chances of encountering a glossy type are always quite low, with the potential hovering around 1 for every 450 encounters.