What's the most efficient way to get Level X Gathering Materia?

[Arohk]

Farming purple scrips?
Farming cowries?
Buying them off the MB?

These all seem burdensome or expensive. =(

You can transmute combat materia to new materia, it is a tidious process but because you get an overabundance of them it might be worth giving it a try.
You need to use 5 different materias to have the bes chance to get crafting or gathering materias.