I have seen these gas bag cars on other media too, I have a feeling that they put a trailer behind buses for the gas bag.
I would imagine that they would have been filled from the gas mains. I know that town gas does not lend itself easily to being bottled, apparently it will only liquify under very extreme high pressure. I have a friend who was a scientist for British gas, who said that they were researching ways to bottle town gas, but the government pulled the plug on all research with the arrival of north sea gas.