Best bet is a flea market. I picked up one at show/flea market. He had 2 and I should have grabbed both. A lot of the old stationary engines had them. I doubt you will find one anywhere else.
This is not rocket science 701.
Get a meter you like.
Mount it where you want it.
Run a wire (any color) to the battery side of the coil.
Run another wire (some other color) to a chassis "ground".
Hook them to the meter.
If the meter reads backward, switch the wires.
Put your tools away and go have a beer.
Pete, thanks for the reply. The problem I been having is no 6 volt gauges as well as 1 pos. term for the instrument, that leaves the housing to be a negative ground. Reverse them and it will read backwards. Also, I bet it would have to be isolated from the chasis ground
Thanks for the reply
All the digital voltmeters I see still need 12v or more for their internal circuits. They can measure down to zero, but need the 12v to drive the display.