Go to the island where the Ancient Arena is, there are islands directly east of it. The first island will do fine. Walk around there and there will be battles that should be easy depending on your level. Eventually you will encounter two very strange creatures called hayokanton (not the exact name, i think), defeat them and they will give a very high amount of money and exp. (very high) use a lucky card to get even more, if you run out of lucky cards, go back to the Ancient arena and fight the first round and then quit you will get a lucky card and a couple of other items. When you fight these monsters Arch Smash, Magnum Fang (for more fast draw attacks, duplicate a secret sign to make every attack 1 mp) work pretty good but then again you might find something that works better.

NOTE* these enemies can be hard on the islands depending on your level and they have strong attacks that have status changing effects (poison,curse,disease, etc. but be aware that it will not be only one status change) use a full Libra or even better duplicate the Sheriff Star from the abyss it will help greatly. One of the best tactics is to focus on one then the other.