This is an in-dept money making guide, there is no information about Gym rematch and NPC rematch as well. If you needs those go to the appropriate page and region for them (Kanto \(or Hoenn\)).
Amulet Coins are a useful resource to farm because this perishable good is often used to buff up the salaries of NPCs and Gym Leaders. You could fetch a good 15k - 22k just by farming this resource. If you can, you should farm at One Island near the Kindle Road (Meowth holds the item) in Kanto region.
I like to farm Leftovers because they are desired which makes them sell well, and for quite a decent amount at 35k-40k each. Only Snorlax can hold Leftovers, he can be found in Cerulean Cave in Kanto region. He can also be lured on Route 16, still in Kanto or Route 225 in Sinnoh.