If you're in need of SP, here's a good way to convert your materia to massive amounts of it. Take any status-raising materia (such as HP Up), and fuse it with a Chocobo Feather or another item that will raise the materia's attributes. Have the attribute added high (which shouldn't take too much SP), keeping in mind that the better-leveled the materia, the better this will work. Once you get the materia, convert it into SP.

By converting a materia like SP++ (30+) fully mastered, I got 150,000 SP. The rarer the materia converted, the more SP you will get from it.