You don't need a specialized grout saw to remove grout from between wall tiles. All you need is a 6d (2-inch) finishing nail and a short length of wood dowel. Bore a small pilot hole about an inch from the end of the dowel, then use a hammer to drive the nail through the hole.
Hold the dowel up to the wall and place the nail point in a joint between the tiles. Then use short downward strokes to carefully scratch out the old grout. If the nail point becomes dull, sharpen it with a metal file.