Been a while I don't watch the series, but it is strongly implied that the Fire Nation killed all airbenders. Of course, since they probably have no armies, they would be easy targets.
But, the nomads seem to have lived in places difficult to reach. If the Fire Nation attacks, the nomads should have had plenty of time to run away (you know, they can fly after all). Fly where? Well, there were plenty of places relatively free from Fire Nation's influence even after the hundred year war, like the Earth Kingdom's Omashu or Ba Sing Se. Of course, they're nomads and wouldn't like to stay and live in cities like these, but I'd rather do that than being massacred by the Fire Nation.
As a side, small note, we also see in the air temples "Chambers that can only be opened by an airbender" that somehow seem to have worked and kept their contents safe from the Fire Nation. Now then, if they didn't want to run away, these chambers look like nice bunkers...
Was there a realistic explanation as to how did the Fire Nation effectively kill the 99.9% of all air nomads?