Sorta depends, for me if I can level and casually find people selling the spells I just buy them. If not I level up research myself. It just takes a couple/few thousand plat to get your research skill high enough for decently priced spells. If you have access to a porter, or like to run around mindlessly, you can scour merchants for the money words and make a little side hustle. Also, its fun to be able to help people who need the combine.