Required: Beaten the Elite four at least one time, Amulet Coin and the VS seeker.

Make Sure you already give the RUBY to Celio on 1 Island to obtain the Rainbow Pass.

Go to 4 Island and Surf North-east until you reach an Island.. Talk with the lady standing on the far west of the island. if this is the first time to visit.. give your first pokemon the Amulet Coin. the one you want to fight with. and beat the lady to get 20,000 Bucks! (Is she mad) however.. you can use the VS seeker to grow your money on this easy way without beating the leaugue all the time (cause your rival will only give 15,000 Money if you use the Amulet coin).

NOTE: and if you already Notice.. there's another Lady on the island.. that will give 14,000 for battle. Good Luck!