On Island 5, there is an area to the northeast called Gorgeous Resort. There are 2 wealthy trainers you can face. They are named Lady Jacki and Lady Gillian. Lady Jacki gives you $10000 if you beat her($20000 with Amulet Coin) and Lady Gillian gives you $9800($19600 with Amulet Coin). The pokemon they use also carry expensive items. Jacki's 2 pokemon carry Stardust, Gillian's 2 Mareep have Stardust, and Gillian's Flaafy has a Nugget. Just use Thief or Covet to obtain these items. and if you use vs seeker they almost always want to fight