You can't.

Elixirs can be bribed from various monsters, and dropped or stolen. The more powerful forms of Elixirs, such as Megalixir and Final Elixirs can be formed when you Mix Megalixirs and Elixirs. Megalixirs can also be found, bribed, and dropped.

Varuna, 1.12 million gil for 20 Megalixirs
Xiphos, 54000 gil for one Megalixir
Elixirs and Megalixirs can also be found in certain Treasure chests throughout Spira

Although, in my experience, I've had 99 Megalixirs more often than I've had Elixirs.