My method in gaining infinite gil in FFX

In battle vs Dingo cast haste on Yuna and then escape with other two characters.constantly guard with Yuna to charge her overdrive.

Then go battle Kottos and Grand Summon Anima.Kill Kottos with Oblivion and after the fight you will get 40 Healing Springs(sometimes you will get 2 Dark Matters but if you do, fight it again).

Then go to Customize menu and select that armor you bought in Killika and scroll down to SOS Regen and learn it .Do this process again for each of the armors.

But dont learn anything except SOS Regen because you will need much more time.

When you are done , sell those armors to the guy standing in front of the monster arena for 37,700 each.You can do this as much as desired

only problem with this is that requires a lot of time but it's much better that Mimic at Omega Ruins