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