In Pixelmon the Galarian legendary birds are obtained through a quest. Here on limitless we have our own method to obtain them.

The legendary bird trio can be found by using a fully charged Orb on a legendary Shrine

To obtain the Galarian forms on Limitless, you need to have Galarian form Pokémon in your party. Each Galarian form Pokémon in your party gives a 30% chance for the legendary bird to be Galarian maxing out at 90%, three Pokémon.

There are 19 total Galarian formed pokemon: Galarian Meowth, Galarian Ponyta, Galarian Rapidash, Galarian Slowpoke, Galarian Slowbro, Galarian Farfetch'd, Galarian Weezing, Galarian Mr. Mime, Galarian Articuno, Galarian Zapdos, Galarian Moltres, Galarian Slowking, Galarian Corsola, Galarian Zigzagoon, Galarian Linoone, Galarian Darumaka, Galarian Darmanitan, Galarian_Yamask, Galarian Stunfisk

They must be the Galarian regional form in order to be counted.