Chimerageist is unlocked by capturing one of every fiend on the Calm Lands. There are nine in total:
Skoll Nebiros Flame Flan Shred Anacondaur Ogre Coeurl Chimera Brain Malboro
But if you want to go the stealing route, then Coeurls can be found all over the Calm Lands. Alternatively, you also get a one-time gift of x99 Mana Springs by unlocking Vidatu at the Monster Arena. Vidatu is unlocked after capturing four of every Imp-type fiend and there are only three of them:
Gandarewa (on Mushroom Rock Road) Aerouge (on the Thunder Plains) Imp (in the Cavern of Stolen Fayth)
Note: there are no bribes available for Mana Springs in the entirety of FFX, including bosses or one-time interactions.
What Are Mana Springs Used For?
When you use one in battle it absorbs some MP from a single enemy. This is actually pretty useful, considering how limited MP can be as a resource for most of the game. You can also use Mana Springs to customize Magic +5% onto weapons and Magic Defense +5% onto armor. Aeons can even use them to learn both Magic Break and the spell Osmose.
Magic +5% and Magic Defense +5% both require x2 Mana Springs to customize Magic Break (Aeon) requires x4 Mana Springs Osmose (Aeon) requires x10 Mana Springs
Rikku can also mix up a bunch of cool stuff with Mana Springs. Some of the better results include: Panacea (Potion + Mana Spring): Cures all negative status effects for all allies. Megalixir (Al Behd Potion + Mana Spring): Fully restores the party’s HP and MP. Yay. Mega Phoenix (Phoenix Down + Mana Spring): Revives all KO’s party members.
Are Mana Springs Worth Farming?
Sure. Chimerageist isn’t too hard to unlock. And having Mana Springs is useful for both MP recovery and Rikku’s mixes. The customizable abilities don’t require too many of them either. I’d say farm fifty or so and that’s probably all you’ll need.