We found Frank's Marinara sauce in our grocery store it is awesome, and only has 1 carb per half a cup. I would not even count it, if you were just braising...or if you are insistent on counting it then count .33 carbs.
I use Hunts Low Sodium tomato paste and mix it with a little chicken broth or water to make it into sauce. It has the least amount of carbs of any of the other tomato sauces/paste. I use it on mock pizza too!