My propane supplier (V-1 Propane) has gauges for 20 and 30 lb.cylinders that are built into the OPD float. The gauge indicates the exact level of the liquid in the cylinder.
The last time I had my two 20 pounders filled, I was going to have the gauge OPD installed in at least one of them, but they were sold out.
Maybe next time.
It is likely that one would have to go to a propane distributor to find these gauges. I doubt that a business which fills cylinders as a sideline would have them, nor would they have the people trained to install them.