Upper level key spheres?

[Tyrion]

Where are they? I've only ever found a handful of level two key spheres, I've never found another, so who drops them? I'm looking for level 2 and above, most of my level one are filled in now.

[Rukk]

At what point in the game are you right now?

[Tyrion]

Just got on the airship.

[Odivan47]

Lvl 2 key spheres are dropped by Tonberry (Cavern of Solen Fayth), Defender Z (Zanarkand Ruins and Omega Ruins), Varuna (Omega Ruins) and some bosses. Most of them drop 1 if killed and 2 if Overkilled. The fastest way is to bribe Behemoth (460k Gil) (Gagazet) for 30 spheres, but I don't think it's necessary.

Lvl 3 key spheres are dopped by Defender Z (Zanarkand Ruins and Omega Ruins) (But it's a rare drop), Behemoth king (Inside Sin), Demonolith (Inside Sin and Omega Ruins), Machea (Omega Ruins) and some bosses. Again, most of them drop 1 if killed and 2 if Overkilled. The fastest way is to bribe Demonolith (900k Gil) inside Sin and omega Ruins) for 40 spheres but this is absolutely not necessary. You can also steal a few from Biran and Yenke in Gagazet (But only if Kimahri knows "steal").

Lvl 4 key spheres are mostly dropped by bosses. The best way to obtain them is bribing Chimira Brain in Calm Lands (98k Gil) for 2 spheres.

Any more doubts just search for FFX item database.

[RiO]

Addendum to the above: don't worry about the gil you'll need for bribing.

By the time you're ready to start grinding for Lv3 and Lv4 key spheres to unlock everything, you'll be ready to tackle the Omega Ruins. The mimics in there give you 50k gil a piece. That's 100k a piece if you beat them while a party member armed with a weapon that has the Gillionaire skill, is actively participating.