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