There are 2 ways to unlock regular Decks:
1. Lucky Boxes : Opening Lucky Boxes with Coins provides a chance to randomly receive Decks and other Items. When you have collected at least 200 Coins, you'll see the OPEN LUCKY BOX button after every time you win. You can also find the OPEN LUCKY BOX button in your Collection. It's possible to collect all regular Decks and Items in Solitaire: Decked Out by Lucky Boxes, even without any in-app purchases.
2. In-app Purchase : The BUY NOW button allows you to buy a regular Deck via an in-app purchase.
For information on how to unlock the "Alone in Space" Deck, please see "How can I unlock the" Alone in Space "Deck?" Question.