Where to Find and Catch Phione
From Pokemon Go Wiki
Phione are found and captured in Nest and Habitat locations.
Nests
Phione Nests are locations that consistently spawn Phione.
Habitats
Phione Habitats are spawn locations where Phione is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: