The premium currency in RollerCoaster Tycoon Touch is Tickets and they should be spent carefully as they are not as easy to earn as Coins. At the start of the game Tickets have two purposes, they can be used to complete missions instantly or to purchase more packs of cards. Unless you are desperate for Coins or Experience Points to level up you should just use Tickets to get more cards.
Unlocking Achievements in the game will earn you the most Tickets. Here are two other ways you can get more of them. If you have played long enough to unlock the ability to promote your park you can earn more Tickets by driving in more customers using the bullhorn icon that is in the bottom-right corner of the screen. Another way which involves spending real money is to tap the 'Offers' button in the store and purchase the products or sevices that Tapjoy offers.