You can get them at the helitours in lower algonquin, or you can spawn an Annihilator (police helicopter) by dialing 359 555 0100 on your cellphone (cheat)
To get in the helicopter, you should simply have to press the "Enter Vehicle" button (varies from console to console, on the Xbox 360 it's the Y button).
Helicopter
A great helocopter
NO
give him the jumbo stuffed catapiller
You have to buy them at the car shop
Helocopter
Yes But No Parachute.
NO! never!
Yes
Annihilator
Under the bridge
You can fly helicopters.