Stamps may be earned by filling orders for Eagle Eye Eddie. He randomly awards bronze, silver, or gold stamps once you complete his order. You can also exchange ribbons from prized animals at the Prize Tent for stamps. A gold stamp requires 10 yellow ribbons, 14 red ribbons and 5 blue ribbons. A silver stamp required 15 yellow ribbons, 7 red ribbons and 2 blue ribbons. A bronze stamp requires 4 yellow ribbons and 1 red ribbon.
You can buy them directly from Zynga using the in game purchasing system. Just click on the + sign next to your stamps balance in the game and you will be offered various stamp packages that you can buy for cash.
This is the article page for stamps. See the category page here for a list of the different types of in game currency and related pages.