Go to the Islands Closest to Heaven/Hell and run around the edges of the islands tapping the X button (or whichever one you have set to confirm). You'll find a lot of hidden draw points with rare magic, including the following:

Tornado
Quake
Full-Life
Aura
Triple
Meteor
Ultima

If you don't want to risk it against Omega Weapon, this is a good way to get all the ultimate spells. It'll probably take you a while, as you can only draw 2-5 of each spell at a time, but luckily the draw points refill pretty quickly, so you can clear out one island, and fly over to the other one and pretty soon they'll be ready to draw again.

Use Diablos' Enc-None ability to speed up the process.