You can mix your own. Get some mortar clay (sometimes called fire clay) and follow the directions on the bag as to the optimal mix proportions for the clay, cement and sand. As to the gaps, why wouldn't you just fill them with the mortar, you can stuff the gaps with it even if you don't have a backing material. The only problem with steel wool is that it's an excellent conductor of heat, so I'd make sure that it wouldn't extend back near anything that could be combustible (wood stud, drywall surface, etc.).