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