You will find the clovers that provide extra lives as power-ups all over the game. Power-ups reappear whenever you leave and re-enter a screen, so once you find a clover, if you need more lives, just leave and come back to get more.
The number of extra lives that you can have at any one time is limited by the number of clover-shaped boxes you have.
You start the game with 2. Read on for the other locations.