Economizers you can get many... I have about 6 economizers in my "all life" save in my Snes catridge of FFVI... I got some of them from the Brachosaurs at the Dinosaur Forest, and the others in the Phoenix cave (where you get Locke in WoR) from the Aquilas. It's difficult to find an Aquila, but when they appear, it's easy to steal the economizer (easier than getting them from the Brachosaurs)... I used to control them with Relm and then steal from them with Locke, with the thief enhancer relics, and specially equipped to hit low hp amounts, so I didn't kill the Aquila until I had gotten the economizer... Curing her if necessary...
It took me a lot of time collecting my economizers, but I love the relic *-* ...

About Gem box, I'm almost sure that you could get more than one, If I'm not wrong, there was some item you could exchange at the coliseum for a Gem box, I don't remember well... I will try to check again in my saves... But I'm almost sure you could get more than one... =/ ...But well... If you guys say you could get only one, I can believe you, but I still have the impression I got a gem box at the coliseum once... =/
Maybe I'm confusing it with something else (even thought is hard to confuse a gem box xD...but well, it's been a while since the last time I played FF6)... I will surely check my saves again... =/