You can get more coins by touching them with your Bubs (balls) during a game.

Each time you touch a coin, it will be added to your total immediately, ready to be used right away. 

Once your game ends, you can also earn a bonus amount equivalent to all the coins you accumulated during that game after watching a short advertisement. 

You can purchase bundles of Coins as well in the Shop by clicking on the shopping basket icon from the main menu.