flutflut
does it cost more ap for each sphere lv or does each level use the same amount? also, are levelspheres dropped or what? an is there limited # of certain sphere types (like level & ability spheres)?
Yes, the sphere levels are treated just like traditional levels, where each additional level requires more experience than the previous. Each sphere, however, only ever costs one level. So, say, your first sphere level requires 100 exp, your second will require 150, your third will be 220, your fourth will need 300, etc.
Sphere levels are obtained just by acquiring the experience. It's just like in other games only instead of reaching level 1, 2, 3, etc, each level is replaced with a sphere level.
I don't think any of the types of spheres are limited availability. I know some of them you won't be able to find a decent supply of until end game, but I think all types of spheres are available through the monster arena in the Calm Lands. Some of them (like Luck Spheres) might be harder to get, but I don't think there's ever a point where you run out of any particular sphere forever.