Hate to ask the obvious, but doesn't John Deere sell them?
If not maybe go to a NAPA or other parts house and look through their books on gauges for generic transmission oil temperature gauges.
Or if all else fails go to a Cat dealer and find the oldest parts guy and have him look up a transmission/torque converter gauge for anyone of many older machines. If you can't find a mechanical gauge, if that is what you have now, an electric gauge should work fine as long as you get the matching sender, just be sure it is for the correct voltage, 12 or 24 volt, of the 550A