You might know this one, eqiup w-item run to a battle highlight a selectable item you have at least two of and click ok three times on it. After that click cancel, ok, cancel, ok over and over again. Each time you do that you should get more and more of the item you highlighted for free.
Another is throw an elixer at the magic pots in the northern crator to be able to kill em.
Underwater, at the sunkin plane, Morph the creatures there to obtain free sources.
master all the materia in a group to exchange them for a piece of master materia which equips. you with all the magic command or summon you exchanged for one. This only works on magic, command, and summon though. Also if you got the blue materia you can get closer to it to recieve behamut zero.