The best thing to earn more money is to slay monsters and collect all the items that they leave. The items could be powerful and thus costs a lot of money. The money you earned could be used to buy much more stronger weapons or to buy potions, etc.
The onyx sword can be found in the onyx tower where the last chapter begins. You can obtain this by killing the large monster on the lower left corner of the castle. It is a very powerful sword which you can use to defeat the final boss of the game.