lv. 3 & 4 key spheres

they're supposedly in abundance in the omega ruins, but i've been fighting for a long time and no dropped spheres, only the common ones. is there a certain area in the ruins where certain monsters drop these, or are the dropped spheres rare occurrences? cause i really need some to get good spells like ultima and full-life. thanks.

Bribe a Demonolith (Omega Ruins or (SPOILER)Inside Sin) with 900,000 GP and you'll receive 40 Level 3 Key Spheres.
Bribe a Chimera Brain (Calm Lands) with 196,000 GP and you'll receive 2 Level 4 Key Spheres.