Have you tried checking with a local leather shop that works on motorcycle leathers. Often they do zipper repairs and may have the part you need or know where to get it.
You mean a reverse slider as in the "flatter" side of the coil zipper is out on the same side as the pull, right? HPG does most of their zippers like that too. OWF has had the most reverse sliders available in the past. I'd try them. I agree that they have been hard to find. Outdoor Wilderness Fabrics - Supplying all your outdoor fabric needs