After completing the main story, you can get level 60 Bronzong and Noctowl to appear in the darker grass in the Abundent Shrine. There are a couple other places where level 60s will appear after completing the main story as well. And as wesley says you can hook pokemon up to level 70 if you fish into one of the "bubbling spots" that appear on the water. However, in all scenarios it is erratic -- level 35's are much more common in those areas.
Volcanora and Kyurem are legendaries that appear at levels 70 and 75 respectively as well, but I don't think that's what you are looking for.