Best way to get Photon Crystals

Yes, not spheres, Crystals. I need a ton to level my Timed Ability crafting. So what's the most efficient way to get Photon Crystals?

The lower floors of Extreme Quests are a good source of Photon Drops that you can then convert into Photon Crystals (10 Photon Drops = 1 Photon Crystal).