You may also be looking for the gold material.
The only reliable way to gain gold is by selling items to shopkeepers. Shopkeepers also have a limited amount of gold to give, so technically there is only a finite amount of gold to be had.
Another way to generate more gold is to beat Huang Ming Pong the sumo wrestler in his sumo arena, after which Richel Decos will provide you with a shirt advertising his banana company. Sufficient time spent wearing this shirt will earn you money from Richel Decos, but the amount is rather small.
Finally, a Wand of Alchemy can be used to transform any item into its price in gold.