Unfortunately to get Charizard in White 2 you will have to transfer from another game where you can get Charizard. Firered, Leafgreen, Heartgold, and Soulsilver are all games you can get Charizard.
Another way to obtain a charizard is to level up a Charmander which you can obtain through GTS negotiations. Here is a Link. It is also possible to obtain a Charizard through GTS negotiations. Credit to theepicME, in comments.