Spirit binding.

[Niwashi]

You will want ilv.45 or ilv.55 items to spiritbind as those will yield rank3 and rank4 materia.

If you're doing it for profit, this can also vary according to the current state of the market board. For instance, last time I checked, on Coeurl, rank 3 materia (of all types) was actually selling for far less than rank 1 or 2. (I'm guessing too many people were trying to SB the level 45 stuff everyone talks about, so the market just got too flooded with it for a while. It's been a while since I checked, though, so I'm not sure if that's still the case or not.)