My propane distributor has magnetic gauge/OPD valves available for small propane cylinders (10, 20, 30 lb.). The gauge snaps on the outside of the OPD valve, and magnetically senses the float position.
I have one on a 20 lb. cylinder. It works great. When I get the other cylinder re-certified I will have the new OPD valve and gauge installed in it.
A propane "filling station" probably will not have the valves. It is likely that one would have to go to a propane distributor to get the valve changed.