There are 3 ways in Kingdom Hearts II to get a Mega-Ether. They are detailed below:
Treasure Chests
There are 2 Treasure Chests in Kingdom Hearts II that contain a Mega-Ether.
This Treasure Chest in the “Agrabah” room in Agrabah contains a Mega-Ether.
This Treasure Chest at “Pride Rock” in the Pride Lands contains a Mega-Ether.
Synthesis
You can also Synthesize a Mega-Ether at any Moogle Shop.
First you need to acquire the Mega Recipe, which can be found in this Treasure Chest in “Beast’s Room” at Beast’s Castle.
Next, you’ll need to synthesize a Mega-Potion. Then you’ll need to synthesize another Mega-Potion, but add a Serenity Stone to the recipe this time to upgrade the Mega-Potion to a Mega-Ether.
Enemies
You can also find Mega-Ethers from enemies. Certain enemies will drop one when they are defeated.
The Crimson Jazz has a 5% chance of dropping a Mega-Ether. Crimson Jazz can be found in Hollow Bastion, Beast’s Castle, Olympus Coliseum, and Agrabah.