Some are included in the starter set that cost $10. The starter set contains 30 cards which contains some summoners and monsters. You can also buy them directly from the market on the official site or win them in giveaways or tournaments. You can collect / trade / Sell without purchase but if you want to play in the tournaments or leagues you first need to purchase a starter pack.

If you are thinking of signing up my referral link is below. I run special Steem Related giveaways for anyone signed up under it regardless of if they purchase a starter pack or not. Use your Steem details to sign in.

https://steemmonsters.com/?ref=rentmoney