Unblockers can be found in the special structures you can find during your journey through the islands of the Special Event. For example, the Greenhouses in the Fairy Kingdom adventure:
After cleaning the path to the object, you will be able to collect a random amount of the unblockers:
You will need those unblockers for clearing the special obstacles you will find during your journey:
After clicking on the obstacle you can see what items exactly you need for clearing it:
You can also receive the unblocker in the Free Gifts once per 8 hours:
And in the Community Gifts, more about Community gifting system you can read here in this article.