Ralts / Gardevoir / Gallade
Ralts can be found by Headbutting trees on Route 34 in the Johto region. There are 7 trees that can be headbutt which are highlighted in the picture below.
You can only headbutt trees once a day. The day resets at 10am UTC/6am ET/3am PT.
Ralts can also be found as an Ultra Rare spawn in the White Forest in Tree 8.