Where to Find and Catch Popplio
From Pokemon Go Wiki
Popplio are found and captured in Nest and Habitat locations.
Nests
Popplio Nests are locations that consistently spawn Popplio.
Habitats
Popplio Habitats are spawn locations where Popplio 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: