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