There are 3 ways to get boosters.
1. When you drop 10+ bubbles together, watch the bar fill up. When it's full, you get a free BOMB.
2. By popping 7 times in a row you will get a FIREBALL.
3. By using your coins to buy either bombs or fireballs.
Note: Boosters can travel with you, only when you purchase them using your coins Otherwise, you can use them only at the level you've received them.
Challenging, is it not?