I would go to the Moblins at the Archylte Steppe. I had the most luck getting them to appear by making it rain and going to the watery place. They will also sometimes drop power crystals. With the upgraded Collector Catalogue I was getting about 5 power orbs and 3 power crystals a fight. Though sometimes nothing at all...it can be finicky. You can also buy them from chocolina after a certain point in story progression.