where can I find one?

Sgt Contrerras in the terminal building head to the concourse and you will find him in the supply shack.

http://www.youtube.com/watch?v=BtyzCnn17QM

Should sufficiently explain it.

great found him thanks...

is there any quick way to the supply shack without traipsing through the terminal each time?

Go through the terminals main doors ( the area with the doctors and Colonel Hsu ), then straight out the doors opposite on the ground floor.
Into a large area with training going on, and aircraft dotted about.
Then just run around untill you find his back door.

It's not really a short cut, but it is slightly quiker imo.