Location near end of game(SPOILER)Zanarkand is great for level 3 spheres, you can get loads there...

As for level 4 you can bribe them from the Chimera Brain for 196,000 for 2. I think the Chimera Brain is a monster found in the Calmlands. If you have captured it in the Monster Arena this should be easier. Radje you should be able to do this as I think you said you had a lot of monsters captured...