The simplest way to substitute 8 ounces of tomato sauce is by mixing 4 ounces of tomato paste with 4 ounces of water. Add seasonings to the mixture until you reach your desired flavor.
If tomato paste is not in the pantry, a stewed tomato substitution is the next best alternative. Combine 1 cup of stewed tomatoes with water, and blend until the mixture is smooth. Season the mixture with salt. If this option is unavailable, use canned tomato soup that is found in any supermarket. One 10 3/4-ounce can of tomato soup yields more servings than one can of tomato sauce.