You can earn Bingo Balls by just spinning! When you spin in any game, you will get a random bingo ball with a number that will match a number in the bingo card. You can find your current Bingo tier and required amount of coins per Bingo ball by tapping on the "Bingo" icon at the bottom, on the left of the "Piggy Bank" icon inside a game. You will get a bingo ball when you spend the required amount of Coins according to the Card Tier.
Figure: Bingo Ball icon