Scag and i believe ferris make a gas and propane mower...very pricy.
It sucks when you run out and have to go to get propane because you didn't have time the day before.
Also, if on a large property and you run out, you have to lug the propane to the mower or drive truck close ...no small portable tank available....
I actually have the Scag unit, it has been great so far and the tanks last a while. They are standard forklift propane tanks, which the propane companies up here have exchange programs for. It is nice having the gas backup on the mower though.