Coins are the easiest currency to get and can be earned. Coins can be earned by finishing quests, completing orders at one of the NPC Markets or from other players at the Farm Stand.
You can buy them directly from Zynga using the in game purchasing system. Just click on the + sign next to your coin balance in the game and you will be offered various coin packages that you can buy for cash.
You are limited to 20,000,000 (twenty million) coins at this time. You can still sell items or complete orders, but your coin balance will not increase.
You can also get coins from the daily Hope Chest, by spinning the Prize Wheel, by filling the Sales Bonus Meter or by trading ribbons at the Prize Tent.