I had a customer in the shop a few weeks ago who had an 88gm Crosman 'airsource' bottle (i hate that name... it's CO2 not air! The clue's in the fact that it's filled with CO2!) and he had converted it (or had had it converted) with some sort of q/f male fitting to convert it to hold air.
I haven't seen him since, i wonder if he's still alive....?