Well my fav. place to level up is the omega ruins... if you have the airship, but don't have it on your list, you gotta like select the search option (when you talk to Cid) and then like... put in the coordinates X-74 Y-36... or just about that... there are big powerful monstas, but most of them give a good amount of experience, and it's a nice challenge and such if you're not already very high in ze levels
As for level 3 and 4 sphere keys... I got my stash of level 3 keys from that fight with those two ronsos on Mt. Gagazet (stealing... My Khimari has a lot of Rikku's skills

)... but you can't go back and do that if you're already past it sooooo *looks at faq*
Also apparently you can bribe them (if you have that skill) from something called a Demonolith in the omega ruins or inside sin... I dunno if that's the Japanese name or the US name... but yeah... they're not very easy to find, me don't think....
as for level 4... says you can steal from Nemisis in the monster arena (on the calm lands... I guess it's one of the area conquest, species conquest, or original creatures there)... And it also says you can bribe it from the Chimera Brain (they're on the calm lands... it's a rare enemy, so it only appears in like the northwest area of the calm lands, I think)...
And if you don't have bribe, then first level up rikku or someone to get it
