Keys are the hardest currency to get but they can be earned. Keys are mainly earned by finishing quests orachievements. It is a very slow process and many people resort to buying them directly.
You can buy them directly from Zynga using the in game purchasing system. Just click on the + sign next to your key balance in the game and you will be offered various key packages that you can buy for cash.
You can also get keys from the daily Hope Chest, by spinning the Prize Wheel, by filling the Sales Bonus Meter or by trading 20 yellow ribbons, 15 red ribbons and 13 blue ribbons at the Prize Tent.