Not sure if I understand your question exactly, But if you`ve got already got a MINI, then have a look at the Tag on the side of the seats which will say AirBag (if you`ve got it).
If you just want to Buy 1 to play with or something, then you`d have to go to the Junkyard I guess with a Screwdriver & a Bullet proof Vest / Head shield
BTW, Did you see Final Destination 2 & How Airbags can make you Jump
