Level 2 and level 3 spheres are dropped by fiends in Omega ruins at fairly constant rate. So, only level 4 spheres can be an issue. But since you can get them at Calm Lands, not a big one, either.
I have 99 level 1 and level 2 spheres. 54 level 3 and some 15 level 4 spheres. They are not that rare afterall. <img src="/images/graemlins/smile.gif" alt="" />