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