Obtaining Hi-Ethers was made much easier in the Zodiac Age version of the game. 
Hi-Ethers can be purchased by seeking out the Baknamy Merchant in the Necrohol of Nabudis. Check out the Necrohol of Nabudis section for more information.
Even after employing the strategies for conserving MP and obtaining Quickenings for all of your characters, it can still be incredibly difficult to keep MP high enough for healing and buffing your characters. There is a method that you can use to farm for Hi-Ethers. It is by no means a fast method, but it is effective if you have some time to spend hunting them down.
This method involves repeatedly visiting a few treasure chests in the Feywood area that have a higher than average likelihood of containing a Hi-Ether. Note that, even with this higher than average likelihood, there is an element of luck involved and it may take several attempts to stockpile some Hi-Ethers.
Circle this area repeatedly, visiting all of the treasure chests identified, until you have replenished your stockpile of Hi-Ethers. This method can take anywhere from 20 minutes to several hours depending on how many you are looking to obtain.
