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