Typically it's natural gas or propane that is run through a converter in real time. I've seen some put an actual hydrogen tank on them.
This company has been one of the leaders in hydrogen fuel cells for well over a decade now. Just been having trouble getting people to adopt them, and trying to fight all the frivolous lawsuits being brought against them by oil companies.
Hydrogen Fuel Cell Technology, Hydrogen Power in Buses | Ballard Power
Another huge use of these is in Japan. They are small, and produce enough heat for electricity for a small apartment/house. Put a natural gas converter in front of it and you have a constant flow of hydrogen with very little waste.