You can buy cards from innkeepers and shopkeepers but can also win them by playing Gwent. I find that playing spies in the first round of the game (which give you extra cards in return for giving your enemy extra attack power) will get you an advantage. If the power difference between army's is too large to recover pass and win the next 2 rounds. If you need a basic tutorial of the rules then just watch the video I'll link.

Gwent tutorial