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