Coins to purchase cards in the shop can be earned in arcade mode (press down while over a stage to switch from campaign to arcade mode). I recommend buying and using the "Greed" and "Magnet" cards first to speed up coin collection. Playing arcade mode on stage 3 will yield the most coins, but it is also the toughest stage to play arcade mode on. Stage 1 is the easiest to play on if you are also going for the "Completely smashed!" achievement, but you will earn the least amount of coins there.