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