Yeah, that was the Gem Box. I've never heard 'em called that, though.

The Gem Box is on top of the Fanatic's Tower. You can find Economizers by fighting Brachasaurs in the small forest near Narshe that looks like a dinosaur. Well, when you approach it from the north, it looks like a brachiasaur, and when you approach it from the south, it looks like a gator head. But anyway. You can also steal Ribbons from them.

If you don't have an Economizer, the best thing to do might be to cast Osmose at least once in your barrage of high-level, high-cost spells like Ultima. You might not be able to make it the last spell, keep in mind, because four Ultimas will probably slaughter everything in sight.