You get a bunch of them for unlocking Ultima Buster, like 99 of them.

If you're using the PAL or International versions, you can farm the Dark Aeons (Yojimbo, in particular). For NTSC or NTSC-J, the easiest way is to take on one of the Area Creations that have low HP and a low difficulty, and kill it until the rare drop is chosen. This'll be about once every eight battles?

The EoFF Frontsite section covering FFX has HP values and a drop listing for the Arena monsters.