Previously, you could only get a special balloon as a reward for a Marathon of Festivals. Still, now you can purchase a balloon decoration for your own diamonds.

To see which balloons are currently available, go to your Township -> click on the festival building.
Choose the "Festival" tab and click on the balloon of your Township.
You will see a new button, "Balloons" click on it and choose the balloon you like the most.
Balloons are expensive because it took a lot of effort for other townships to earn balloons during the Marathons. After all, the more difficult it is to obtain an item, the higher its value.