You can either pick one up or at least order one through Tractor Supply, or a Farm Supply store in your neighborhood. There are a number of 6 volt Positive Ground tractors out there, here is a gauge for one.
http://www.steinertractor.com/JDS1910-6-volt-fuel-gauge
The problem you will find is that most marine tanks (Like the one that I think you are using

) make use of a 35 to 240 Ohm Sending Unit. You can check it with a basic Ohm meter without pulling the sending unit, just hook up your ohm meter and put the tank on its end so you can tip it to make the float travel it full length. You should see 35 Ohm's at one end of the swing and 240 at the other.. There are other ranges like 0 Ohms to 30 Ohms, but most Marine applications use the 35 to 240 Ohm range. So you need a 6volt gauge that is matched to that Ohm range. That will be a challenge.
Here is a list of guage manufacturers and the Ohms used:
SW - SW gauges - 240-33 Ohms
VDO - VDO gauges - 10-180 Ohms
GME - GM up to 1964 - 0-30 Ohms
GML - GM 1965 and up - 0-90 Ohms
ORG - Early Ford & Mopar - 73-10 Ohms
I think you most least expensive option (at least time wise) would be to pick out the gauge that you want, and then change out the sending unit in the tank. They can be had for about 40 bucks.
These units fit into the tank, and you cut the arm support (not the arm) to fit the depth of your tank.
http://www.ebay.com/bhp/universal-fuel-sender
The sending unit cares not what your voltage is. Only the gauge does.