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