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