How to make 4-20mA Sink Generator to Simulate with PLC or other 4-20mA Instruments?
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. A simple analog circuit.
Many 4 - 20 mA inputs have a 250 Ω shunt resistor to convert 20 mA to 5 V. If this is what you are dealing with then you can simulate the transmitters with a PP3 style 9 V battery, a potentiometer, a limiting resistor and a multimeter in mA mode. It's very cheap and should suffice for testing.

With R1 wiper at bottom the current will be 9 / (2k + 200 + 250) = 3.7 mA.
With R1 wiper at top the current will be 9 / (200 + 250) = 20 mA.

Decrease R2 to 180 Ω to allow a little more current to ensure that you can reach 20 mA with a deteriorating battery.