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