I don't think there are more than four or five Lv. 4 Key Spheres that you can find in the game. But I believe you can bribe a Chimera Brain in the monster arena. I think you get 2 for about 200,000 gil.

You can also get Lv. 2 Key Spheres from the monster arena. You can bribe a Behemoth and get 30 for about 500,000 gil.

If you are running low in money, I personally would suggest finding and activating Rikku's Godhand, it has the Gillionare ability, i.e. rolling in money.