You can purchase cards by buying a Box which is a collection of multiple Packs of a specific Rarity.
Go to Home -> SHOP to check availability and prices. Tap on BUY to add a desired Box to your Inventory. Make the payment and you're all set!

For e.g., if you buy a 5 Pack Rajat Box, you'll get 5 Packs with 5 cards each with at least 1 Rajat card in every pack.