Question

Where can I find cobalion in Pokemon black 2 after I beat the game

Answer 1

You find it in route 13 at level 45, which can be obtained before the Pokemon league. So you dont need to defeat the Pokemon league. Just go to route 13 from the lacunosa town side and it should be there. If you defeated it or ran away just go back to the same location after you defeat the Pokemon league and then it will be there, this time at level 65.
Source: strategy guide

Answer 2

http://www.youtube.com/watch?v=IilKkkx6nTM

You can catch it at level 45 on route 13.