I was thinking the same thing about helium being a finite resource, and I am suitably impressed by the knowledge provided by the above posters.
Hydrogen used to be the buoyancy gas of choice, and it is relatively simple to make, but the flammability issues, of course are too much of a problem.
I do wonder, though, if some kind of Hydrogen/Helium hybrid could be made to work. There must be a safe dilution at some point.
I cant be the first person to suggest this, so I'm sure somebody can put the idea straight.