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