What's the source of electrical energy in galvanic cells?
Judging by your above comment to beginner's answer, you seem to be confused about how reactions proceed in general.

Take a look at this graph : http://en.wikipedia.org/wiki/Activation_energy#/media/File:Activation_energy.svg

The oxidation in the beginning certainly requires energy, but at the end of the reaction, the system ends up at a lower energy state overall. This causes the reaction to be "spontaneous".