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