Mega bosses are random spawns and there's pretty much guaranteed to be one in each biome including Mushroom Island. It's only a matter of time before one spawns, else you can buy something called a Keystone from other players that when used, will give you the Mega Band used for Z-Crystals and Mega Evolutions.
In the future can you please start a new post with your issue, this way it is easier to be found.
Is there anyway you can make a ticket on our Discord?
We can help you quicker on the discord!
We will need your:
Proof of purchase: