Finding yourself some fusion core to power up that power armor is not that difficult but if you so happen to be in the area, here are some of the places where you can stock up on those fusion cores.
At the Kellogg House, you will be able to pick up 4 fusion cores. All you need to do is head to the desk, crouch down and press the red button. The keys to the Kellogg House will be given to you after you complete the third quest.
At the castle, eliminate all aquatic opponent and you will be able to pick up a few more fusion cores. At Ford Hagen, head to the western edge of the commonwealth and you will find a few more cores at the generator room.
There are also some fusion cores to be found at the Weston Water Treatment plant.
