I bought a little 4-20mA source on eBay for under $10.
The device has a backlit LCD display, and a rotary encoder with steps of 0.05mA (or 0.3125% of 16mA). The current setting can be set as power on default by pressing the knob. It is supplied with a 250Ω resistor which could be used as a load resistor in projects delivering 500mV FSD.
Differently to some similar devices, this is a two wire device, it does not need a loop -ve connection and of course the total current drain is exactly the loop current.
Accuracy of the test subject is well under 1%.
The test captures the response in a project that supplies the loop voltage from a LM2577S boost converter (above) with 3.7V input in this case.
The yellow trace is the converter control signal, cyan is the loop +ve supply, and magenta is the load voltage (proportional to loop current). The signals are a bit noisy, probably mainly due to the breadboard lash up.
The loop current stabilises by the time the +ve supply reaches about 15V and there is about 3V across the load, so about 12V needed for the source alone.