That's how the benders should be able to do. Air is everywhere, so its possible to bend it any time and anywhere on earth .. So is ground, .. even the sea beds are made of soil and stuff .. Water is almost everywhere, there are water molecules floating everywhere, which makes the humidity .. (and clouds)
But the problem is with fire .. no its not available everywhere, but yet the fire benders are able to create them from nowhere, when in reality they should only able to bend the existing fire not create them ... that's where the problem is.
I think firebenders can create fire because although in a fantasy setting it's an "element", it's also a chemical reaction of combustion. I think they just need oxygen to burn. In what situation would they not have oxygen? I'll tell you which one, one in which not being able to firebend would be the least of their worries. I think that's the thing with firebenders. That's why pro-bending has these adapted "earth discs" and water supplies but no torches. Firebenders have always been like that, and it annoyed me a lot when they needed lit fires to use it in the live action movie. Although it wasn't the worst thing in that movie by a long shot.