not quite the most succesful and easiest method is to bribe chimera brains in the calm lands. bribe one 245,000 gil for 1, 2, 3 or 4 spheres. with 2 in each battle that means u can get 8 in one battle. chimera brains are most common behind the travel agency in the calm lands. if u need money, kill the mimics in omega ruins with the gillionaire ability (rikku's CW) for 100,000 gil.