"These vehicles must be equipped with special license plate"
"I checked at our local courthouse and they dont have a clue about any special plate"
That's because there is no "special" or "new" plate. The rule reads....
(c) Is equipped with a special license plate or other designation by the State in which the vehicle is registered to allow for identification of the vehicle as a farm vehicle by law enforcement personnel;
A common Farm Plate will do the job.
There are some changes with Map 21 but farmers had most of these exemptions already.
But keep reading. Map 21 also increases funding for things like enforcment of fuel tax evasion.