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