The trouble with buying a box at the airport, is that on many occasions, you'll phone ahead to find out you can buy a box from the airline in question, only to arrive at the airport and be told that they don't have any right now. That's happened to me on three occasions, which is well over 50% of the times I've tried that option. These days I just don't rely on it. I'll source the box at a bike shop nearby, and make other arrangements to travel to the airport with the bike already boxed, depending on what the city in question has available.