A free available Power-Up card will have sparkles around it (F).
Selecting a Power-Up with a Plus sign and no sparkles (G) will bring up the option to purchase additional ones. Purchased Power-Up cards will not have sparkles around them and will not prevent you from acquiring a free card.
Select the chest for special deals on Power-Ups (L1).
If you are enjoying the game, please take a moment to rate the game when the option pops up.