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