I suspect that being opposite of crystals, and clusters as well, they dont drop from atomos mobs at all. Instead i suspect they drop off garleans. Didn't old gramps mention the VII division being somewhere in mor dhona? Perhaps they really are there and they hold the clusters we need?

Come to think of it, perhaps any normal mob drops it in mor dhona, as i remember another quest npc a while back mentioning how many crystals were losing their aspect there, due to a strange influence (dalamund.)