You can't it can only be found in Pokemon White but you can trade it over if your friend has the opposite version of you.
Rufflet and Braviary are exclusive to Pokemon White. You'll have to trade.
You can not catch Braviary. You have to catch Rufflet in Pokemon White and evolve it.
You can't get Thundrus, Braviary, Ruffet
There isn't anywhere that you can catch braviary in Black. However in White you can catch Rufflet and evolve it and trade it to black.
You have to get it in a trade. Rufflet and Braviary are not available in Black.
petilil, throh, zehkrom, rufflet, braviary, solosis, and their evolve forms.
You can't catch it until you trade someone using white and he/she has a braviary.
rufflet
He can be caught on Route 11, and around Village Bridge. Though if you want to obtain one earlier, you can catch it's pre-evolved form Rufflet on Routes 10 and 11, as well as Victory Road and around Village Bridge. Braviary/Rufflet are exclusive pokemon to Pokemon White version and can not be obtained through normal gameplay in Pokemon Black.
Braviary (as well as it's pre-evolution, rufflet) does not appear in the wild in Pokemon Black version. You will have to trade with somebody who has Pokemon White version. Likewise, Vullaby and Mandibuzz can only be caught on Pokemon Black.
Rufflet (Pre-Evolution of Braviary) is only available on Pokemon White Version
Cynthia in Undella town