Energy Crystals are much easier to get than Power Generators. There's a 4.7% chance of Mugging a Power Generator, and it's best use is to teach Quistis Ray Bomb.

You can find a Power Generator in the Lunatic Pandora if you do something during the Laguna dream sequence. I don't recall the specifics, but it's to do with tampering with the locks on the trapdoors.