Neue schule makes one. It's pricey but I really like how they design the links on their Waterford. I order my NS bits from the UK (better price usually) but if you really prefer to buy in the US, the Cheshire Horse and AllEnglishTack stock that bit. http://cheshirehorse.com/Waterford-J...it-P12286.aspx
I have the Happy Mouth American Gag from Dover: http://equestrian.doversaddlery.com/...32yulzo55would that work for you?
I use it for the jumpers, and it has been a lifesaver. My horse is a freight train, but also very sensitive (i.e. would go backwards with too much hand).