After being ripped off at the local stealership for years regarding the cost of air;
I am an convert to buying online from the USA. With our dollar running high at the moment, and the postal costs reasonable, although sometimes the shipping can be delayed at customs, so you wouldn't want to be 'holding your breath' waiting for the air.
I have bought air from the Europeans once or twice, but found it couldn't handle our hot conditions out here; although, with expansion, you get more for your dollar
Oh yeah, I almost forgot...... avoid the cheap chinese imported air... polluted to the sh*thouse.