The laser amplifier works by allowing incoming photons to stimulate emission from excited electrons in the gain medium.
The simplest gain medium one can describe is that of one with two energy levels, an upper and a lower energy level. Amplification occurs when an incoming photon allows an excited electron in the upper state to stimulate emission downwards and create an equal copy of the incoming photon. An incoming photon on the other hand may also be absorbed by the gain medium if there is a lot of un-excited electrons in the lower energy state. This is not good for our laser action.
So what is done to excite electrons is that in a conventional gain medium the gain medium is 'pumped'. We can now consider a three or four level system where we have a ground level, a higher one and one in between. Incoming photons may excite electrons from the bottom stage to the uppermost stage, where they will quickly de-excite to a middle energy state. Incoming electrons may then stimulate these middle-electrons to provide a laser behavior. In order for these systems to work, and for the medium to not absorb but to instead amplify we need to have more electrons in higher states than in the lower states. This is referred to as population inversion.
So to answer your question, in a conventional laser today the energy comes from another light source which in turn acts as the pump. Nowadays there are laser diodes and these are electrically pumped, meaning that a current or voltage through/over the diode provides the energy to move electrons into the right states. Previously with for example a HeNe-laser, a gas is pumped/excited with an electrical current. To summarize, yes, the energy comes from somewhere else.
This got longer than i expected, I hope I've done no error anywhere.