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