Gamestop is apparently going to have a promotion featuring Shiny Eternatus, as seen on TCG fansite Pokebeach. (you need an account to see links) Most likely it will be like the shiny Zacian and Zamazenta ones where you pick up a card with a code, but we'll have to wait and see until more details are revealed! I'm excited to get the last one to complete the Galar trio.
Here is what it looks like if you haven't seen it!