Power Crystals are actually fairly rare and the monsters that do drop them do so infrequently.

Your best bet is to go to the Archelyte Steppe and set weather to rainy. Mad Frogs (which will spawn on the lake) drop them, Goblins (which prefer to spawn in the grasslands) will drop them, and Fomoire (who is a Cieth Miniboss that spawns near the gate out in the drylands) will drop them.

Fomire is a pain if your party is unprepared (he can be tough and is a hard hitter) but has the highest drop rate of the three mobs mentioned. However, he will not drop more than one at a time.

Good luck!