Here is one source:
www.mypilotstore.com/mypilotstore/sep/1177
It's easy to calibrate. I have the FuelHawk shown in the link; the calibration for my 337G with extended range tanks is shown below :
Mark Gallons (Usable)
0 31.8
1 37.8
2 42.7
3 46.1
4 49.4
5 52.5
6 54.8
7 57.0
Ernie