How do you get bombs in this game?

You need 8 or 9 of 1 colour to make a bomb while playing. You can earn them from rewards in game or buy with the coins if you want to start a round with one already placed.

Honestly it's basically by chance. You have to get at least 10-12 of one color together to make one. It really sucks you can't buy bombs outside of those weird "Sales" they have that they claim are always "66%" etc., off - but we have no way of knowing whether we're actually getting a deal or not since we have nothing to compare the prices to. Smart but shameful lil scam..