Economizers can be stolen from the Aquila in the Phoenix Cave and are dropped by Brachosaur in the Dinosaur Forest. You could also wager your Gem Box for an Economizer at the Coliseum, but that would be incredibly stupid.

There is no way to obtain more than one Gem Box in the SNES and PSOne version of FFVI. But in the GBA version, you can steal a Gem Box (called a Soul of Thamasa in the GBA version) from the last Glutterns in the Soul Shrine.

You can also play around with Relm's Sketch bug in an attempt to gain hundreds of either of the items, but there's no guarantee of what you'll end up with, or if any of your saves will be erased.

EDIT: My hosting site is in the process of moving servers right now, so the links for the enemies might give you a 404 error until the move is completed.