1. The easiest way is to defeat the Elite Four and Champion Lance. Each Elite Four member will give you 1,000 coins when defeated, and Lance will give you 2,000 coins when defeated.
2. Another way to get coins is to win battles against the trainers in the game. Some trainers will give you coins as a reward for defeating them.
3. You can also get coins by selling items at the Poké Mart. Some items will sell for more coins than others.
4. Finally, you can also get coins by playing the coin flip game in the Celadon Game Corner. If you guess correctly, you will win double your bet. If you guess incorrectly, you will lose your bet.