A guy who used to go to my school before he moved had a custom mouthpiece from a dentist that cost him a lot of money, yet it still only fully protected his upper teeth. I bought one from a sporting goods store that protects both my top and bottom teeth and has a hole in the center to breathe through. I molded it to my teeth by boiling it and biting into it. It may not be as perfect a fit as my friends was, but his obstructed his breathing, which resulted in him not sucking it into place and biting down on it untill he was about just about to do something, which telegraphed his moves, and left him unprotected whenever he needed to breath freely.
I'm not sure if there are dentist-molded models that have the whole in the center, but if not, I would definately recommend one with a whole to breathe through. You can find a variety of brands online such as BrainPad, and models that cost a little more and come with larger dental work guarantee if they fail to protect you.