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