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