After the two worlds have melded together, and once you've acquired the Black Chocobo from Mirage Village, head toward the Phoenix Tower. Inside the Phoenix Tower, you'll encounter an enemy called Disabler. Make sure at least one of your characters is a Thief, or have the "Steal" ability equipped, and then begin stealing Wall Rings from every Disabler you encounter. Repeat the process as much as you like and then head to any town to sell the Wall Rings. One Wall Ring alone will sell for 10,000 GP!

You can encounter Disablers anywhere inside the Phoenix Tower, but it's best just to stay on the 1st floor, because you'll have easy access to the entrance where you can rest and save outside the tower.