A megalixer will restore all HP and MP for all party members. So it is one of the best items in the game, and therefore is more rare to find.
I obtained 2 megalixers from the locked silver chests after I obtained the key and airship. I believe I got them in the Nive area from the start of the game. It is located on an island to the east. I did make a note of the 2 chests there which are:
Nive Harbor - Inside the house.
Nive Village - This is on the West side from the entrance
The Nameless Village located on the Isle of Misfits to the south west contains a merchant in the east house that will sell Megalixer for 9800G each