i'm trying to source a replacement flow gauge as per this image
but i'm not entirely sure the best place to go. From what i've found so far it would seem cheaper to just buy a complete replacement regulator assembly.
x3 as unless you found the exact gauge from the same batch or make of regulators it might not even show an accurate reading.
As it is a pressure gauge with the scale calibrated and printed to read flow so is unique to the regulator to witch it was fitted.