Farm Bucks are a secondary currency in FarmVille 2 that can be used to purchase items that you can’t get with Coins.
To get Farm Bucks, you can use Credit Card, PayPal, Bitcoin, your Mobile or through the use of Game Cards.
How to purchase Farm Bucks:
How To Redeem using your Game Card: 
Facebook Credits
Usually, Facebook Credits won't appear from the payment options when purchasing Farm Bucks. This will happen if you currently don't have any Facebook Credits to use in the game. However If you have enough Facebook Credits for a certain purchase, it will appear in the list of payment options.
- Click the “Add Farm Bucks & Coins” tab at the top of FarmVille 2.
- Select how many Farm Bucks you want to purchase.
- Select the payment method.
How To Redeem using your Game Card:
- Click the "Redeem" tab at the top of FarmVille 2.
- Select the desired Game Card.
- Place the Pin Code which can be seen from the Game Card.
Facebook Credits
Usually, Facebook Credits won't appear from the payment options when purchasing Farm Bucks. This will happen if you currently don't have any Facebook Credits to use in the game. However If you have enough Facebook Credits for a certain purchase, it will appear in the list of payment options.
NOTE: Any remaining Facebook Credits that were not used during a purchase will be added to your Facebook Credits balance and can be applied to any Zynga game!