There are four different kind of vouchers: green, blue, purple and golden ones. You get vouchers when you complete a boat delivery (starting from level 20) or when you complete a special truck delivery (starting from level 24). In addition, you can find vouchers in red tool boxes and you can win them playing with the Wheel Of Fortune. You can also buy vouchers with diamonds.
You need vouchers to buy pets and special decorations to put in your farm.