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