There are treasures scatter throught the world and assuming you haven't already collected them, you can simply walk up and grab them. Check these areas:

First, in Trioa Castle, after defeating the Dark Elf, you will be granted access to the treasure room. In the room, you will find 18 chests, one with 5000 gil, the another with 50,000 gil!

Ok, now, get in the airship and go to the castle on the small island far southwest on the overworld map (Eblan Castle). In the castle, west tower, second floor is a chest with 10,000 gil.

Now fly to Mount Ordeals, then fly north. When you see a small town on a small island, land and enter. Behind the bush in front of the weapon shop is a hidden chest with 5000 gil.

For completing all of the map of B4 in the Sealed Cave, you are given a Megalixer, this can be sold for 300,000+ gil. The chances of you actually using it in battle are very slim, so I recommend selling it.

If you have already done all of the above, then the best thing to do is to go to Eblan Cave B1 and just roam around, preferably with the Treasure Hunter augment equipped, and fight the enemies. A common enemy, the Steel Golem, has a 20% drop rate for Gaia Hammers (that would be 40% with Treasure Hunter) and a 40% chance that you can steal one as well. They also drop Giant's Gloves, which can be sold for decent gil.