Do you just want a metal mouthpiece to have a metal mouthpiece? I mean, if your teacher said no metal, he probably means it for the best. Yes, many people sound great on metal mouthpieces on alto. But for most people, it's harder to learn to use a metal mouthpiece WELL on alto. I'm not saying it's a totally different ballgame, but they are pretty different. Even some of the metal mouthpieces I've tried that had no baffle have sounded very bright. And I play on a really dark horn. You wouldn't think so by looking at it, but once you play it, you'd think it was the greatest classical saxophone ever made.
Anyway, I'm not trying to sway you from getting a metal mouthpiece ... but maybe you should try out a ton of mouthpieces before thinking that metal w/a dark horn is the way to go. CountSpatula had a good point about it being the player that makes a good sound ... not the setup. I have always been a firm believer with that. If you've got a decent enough setup that is versatile enough, with enough practice you can achieve anything you want.