In RSE, what's the fastest way to get 12000 coins? I already beat the champion, if that matters.
Buy them at the Game Corner, maybe?
As for where to get the money, there's a Rich Boy that can be rebattled just outside the south entrance to Petalburg Woods, who only has a Linoone and pays out p7,200 from the fourth rematch onwards. The Linoone holds a Nugget too, which can be stolen if you have Thief. There's also a Lady with the exact same properties and payout north of the forest. Petalburg Woods can be traversed north-to-south without stepping into grass, so you can fly to Rustboro City, go back-and-forth to Route 104 until both trainers want to rebattle (check the Trainer's Eyes), go down and rebattle them, and then fly back up and repeat. Bring a Pokémon with Thief and an Amulet Coin, and you can make a cool p38,800 per cycle.