And if you really don't feel like going trough the Elite Four for whatever reason, you can always call up Gym leaders, and fight them again. Along with Amulet Coin, each gym leader is about 13000-15000 ish. Besides Blue, which is a lot more.
I also faintly remember something about someone that appears in an underground tunnel in Goldenrod that only appears on certain days, that sells the items that are worth a lot to you, for less than their worth.
ex. Nugget / But I don't remember what day, if you took the time you could make profit.