the source current is the current that flows from the power source.

A current source produces a constant current no matter the variation in voltage.

Simple source of direct current are Batteries.

Take the internal series resistance of the voltage source and make it the internal parallel resistance of the current source. Then compute using Ohm's law the current of the current source to be equal to the maximum current the original voltage source could supply a short circuit load.

Note: the two sources are equivalent.

The maximum current that can be drawn from a voltage source is dependent on the impedance of that source, the impedance of the connections to the source, and the energy available from that source.