Not sure what you mean by "buy" a ticket. If you mean buy, as in pay for by EPS, a green ticket; the only way I have ever seen this achieved is at staff travel.... after suitable grovelling.....
If you mean, print out a pink flown ticket, then as Humber10 said, you go to the Airport Staff Reporting Centre on the Office levels between Arrivals and Departures. You enter a "death star" type long grey corridor and walk about a mile until you reach a door that has the paint worn off it, where a brazillon hands have whacked against it.
The good news is they have computers and a ticket printer: the bad news, it's such a soleless place, you will yearn for the delights of " Kitty City"
