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