If you're looking for one with a 3-button controller like the one that comes with the phone, with a volume up and down and middle buttons, I haven't been able to find one besides the one that comes with the phone. The Apple 3-button compatible headsets just don't work with the MT4G.
What I have found are ones with one button controllers in which the button works just like the middle button on the stock headset. I have two, a Meelectronics M9P and a Maximo iP-HS5. I suspect all the one-button models will work.