AFAIK they are usually 3.3ohm.
5 watt sounds very high, I can't imagine the airbag to be that high, are you sure it wasn't 0.5watt? If so then a commonly availiable 1 watt 3.3ohm resistor will be fine.
The resistance is the important bit, as long as the rated wattage is the same or above what the airbag would draw, then that will be fine.