Jumbo Flan in the monster arena drop Magic Spheres.
You can bribe Master Tonberries for 960,000 Gil for approximately three Pendulums, or fight Neslug about 15 times (that's with the Overkill).
Nemesis will drop Warp Spheres, Dark Matter, and Winning Formulas. The equipment he drops have Break HP Limit, and I believe some of his weapon drops have Triple AP.
Greater Sphere will drop Luck Spheres, will Earth Eater drops Fortune Spheres which are needed to activate Luck nodes.