To get a free Shiny Mareep in Pokémon Legends: Z-A you must complete Side Mission 17: A Shiny Mareep? This will unlock after you return Lida’s belongings to her and battle her during Main Mission 8: ...
Blue Flabébé can be found among the bushes on one of the rooftops in Rouge Sector 6, directly beside the Académie Étoile.