I buy my tube in large quantities wholesale, but in a pinch I get small tubes from McMaster too. Not always the best price, but they ship fast. Sometimes they are slow to raise prices too, so if tube prices shoot up McMaster is suddenly the best deal in the US.
They don't know how to pack though, if I order 60 feet of 6" tube, more than half of them will have ends that are cracked from poor handling.
They will exchange, but if I did that I'd spend all my time shipping tube back and forth.
For short sections, they should be able to pack it better (I would hope).
Zeph