I've seen videos on line of people getting shiny Mew in a variety of ways, but am not sure if this is legit.
I do know that if you have a Pokéball plus, you can only get 1 Mew, this is because once you upload the data from the ball to the Switch, it is no longer in the ball, then if you soft reset to a pint before you uploaded Mew, it is lost from the game, because the data has been removed from the ball you can not upload another.
So it looks like you have to find a ball that has a shiny in, which could be expensive.