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.
This is the article page for coins. See the category page here for a list of the different types of in game currency and related pages.