Loop Calibrators
Yes, there are several brands of loop calibrators, also called signal generators. The Transcat Transformation is another well-known brand. Just recently, I bought a new Fluke 789 Process Meter. It has all the usual multimeter functions, plus the 4-20 mA generator. It will act as the loop source voltage, or allow using an external source. It also a 24 volt dc power supply. Price is about $650 U.S. dollars.