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