Black Version:
* Zekrom: Found on the roof of the Nimbasa City Pokémon Center after defeating the Elite Four and becoming the Champion.
* Reshiram: Found on the roof of the Nimbasa City Pokémon Center after defeating the Elite Four and becoming the Champion.
* Cobalion: Found in the P2 Laboratory in the Undella Bay area after completing the P2 Laboratory Mission and finding the relic (it's a small mission that involves the team Plasma).
* Terrakion: Found in the P2 Laboratory in the Undella Bay area after completing the P2 Laboratory Mission and finding the relic (it's a small mission that involves the team Plasma).
* Virizion: Found in the P2 Laboratory in the Undella Bay area after completing the P2 Laboratory Mission and finding the relic (it's a small mission that involves the team Plasma).
* Landorus: Found in Giant Chasm in White Forest after defeating N at the Abyssal Ruins.
* Thundurus: Found in Giant Chasm in White Forest after defeating N at the Abyssal Ruins.
* Tornadus: Found in Giant Chasm in White Forest after defeating N at the Abyssal Ruins.
* Genesect: Can be obtained by trading a Chespin from Pokemon X and Y to Pokemon Black 2 and White 2.
White Version:
* Reshiram: Found on the roof of the Nimbasa City Pokémon Center after defeating the Elite Four and becoming the Champion.
* Zekrom: Found on the roof of the Nimbasa City Pokémon Center after defeating the Elite Four and becoming the Champion.
* Terrakion: Found in the P2 Laboratory in the Undella Bay area after completing the P2 Laboratory Mission and finding the relic (it's a small mission that involves the team Plasma).
* Cobalion: Found in the P2 Laboratory in the Undella Bay area after completing the P2 Laboratory Mission and finding the relic (it's a small mission that involves the team Plasma).
* Virizion: Found in the P2 Laboratory in the Undella Bay area after completing the P2 Laboratory Mission and finding the relic (it's a small mission that involves the team Plasma).
* Thundurus: Found in Giant Chasm in White Forest after defeating N at the Abyssal Ruins.
* Tornadus: Found in Giant Chasm in White Forest after defeating N at the Abyssal Ruins.
* Landorus: Found in Giant Chasm in White Forest after defeating N at the Abyssal Ruins.
* Genesect: Can be obtained by trading a Chespin from Pokemon X and Y to Pokemon Black 2 and White 2.
Important Notes:
* Zekrom and Reshiram are version exclusive. You can only find one in each game.
* Landorus, Thundurus, and Tornadus are found in the Giant Chasm only after defeating N in the Abyssal Ruins.
* The Swords of Justice (Cobalion, Terrakion, and Virizion) are found in the P2 Laboratory, but require completing a side mission.
* Genesect is only available in Black 2 and White 2.
I hope this information is helpful! Let me know if you have any other questions.