Where do you get air balloon in Pokemon white 2 plus?

You can't get an Air Balloon in Pokemon White 2 or Black 2. It was introduced in Pokemon X and Y.

The Air Balloon is a held item that boosts the user's Flying-type moves by 1.5x, but it breaks when the user is hit by an attack.

If you are playing Pokemon White 2 or Black 2, you can find other held items that might help you with your Flying-type Pokemon. For example, the Bright Powder increases the user's evasiveness, and the Choice Scarf boosts the user's Speed.