There is a dark counterpart for every Aeon in the game. So that goes from Valefor (found at entrance to Besaid) to Magus Sisters (found near the place where you fight Sin (with seymore in the party))

I can't be arsed telling you how to get to them all - you're gonna need to do a lot more levelling up before you even consider fighting them.

You don't get to keep them but they drop armour with ribbon/break hp limit so it's worth killing them a few times over (i.e. if you don't get what you want then reset and try again) to get some great armour.

The best tactic is simply to level up. Get to the monster arena and start boosting your stats. Max strength, max agility, max defence and you'll be well on your way. You can probably take out a few with just those stats full. But you could do with getting evasion, magic defence and hp cranked up a lot.

Takes time mind.