Airport Extremes use the MC Card connector. You could use any antenna you want as long as you get the right pigtail for it (and the impedance matches, which I think is 50 ohms here). I opted for a 5.5db flat blade antenna ($15), but there are many other options over here: http://sharperconcepts.zoovy.com/category/24ghzantennas/
The connector coming off the existing internal antenna is going to have to be pushed back into the notch it comes out from. You'll quickly realize it doesn't go back far enough and obstructs the little metal flap from closing back. We'll cut part of that flap off.