As a capacitor is charging in a circuit with a battery as the power source where is the source of electrons coming from?
In a discharged capacitor, each of the two plates of the capacitor is electrically neutral. Neutral means that there are for each atom of the plates there is an equal number of positive protons in the nucleus and negative electrons in the hull of the atom. Thus the number of all the positive and negative charge carriers inside the plates is in ballance.

Now if the capacitor is changed, for every electron that is removed from the positive plate, another electron is added to the negative plate. The battery is only "pumping" the electrons from the positive plate to the negative plate.

If a charged capacitor is discharged over a resistor, electrons are flowing from the negative plate through the resistor to the positive plate. Discharging is finished if both plates are electrically neutral again and no more current is flowing through the resistor.