A simple current source can be created from a voltage source and resistor so long as values are much higher (an order of magnitude) than the output circuit.
Note how the values of the voltage source and resistor are much higher than the resistance of the potentiometer. Run the simulation and adjust the potentiometer's wiper position - the current never varies more than 0.1% (0.1uA). The high values of the current source make the potentiometer's value irrelevant, because I = V / (R + Rload)
