Once you've gotten to PetalBurg in the First Chapter you can use a trick to make easy money. First go to RougePort and buy as many sleepy sheep as you can carry, making sure to fill up your storage box aswell. When your pockets and storage box is full, head over to PetalBurg. Once at Petalburg, go into the Item Shop to sell your sleepy sheep for a 2 coin profit per sheep. Make sure you remember to withdraw and sell all the sleepy sheep you stored. To increase productivity, on the way back to rougueport, stock up on turtley leaves, again filling up the storage box, and sell them at the item shop where you picked up the sleepy sheep to gain 2 coins per leaf.
Hint: After getting Taste T. you can cook the leaves into koopa tea for an extra 1 coin per leaf.