Not to be negative, but you have to accept that you are headed uphill. I have a few less common handguns too, and I would not part with them, but I know that finding parts, accessories, magazines, and holsters is almost impossible.
You did not say what you want the holster to do for you. Is this for concealed carry, or for hunting, or for showing off at a barbecue?
It sounds like you have been looking. Did you try ebay? A few folks still sell some gun stuff there (mystery unsolved on that one) and sometimes have rarer items.
Another choice is a generic type holster like an uncle mike's. I think they still make the cordura nylon style with numbers 1 thru 5 that fit most guns. Not an exact fit. I know in the old days people used to jam handguns into whatever holster came closest. Kind of a bad idea, but leather stretches and then keeps the shape. A GI style flap holster for a 45 is probably a close fit to a Witness. I've used one for a HP-35 and they fit fine for the purpose of carting the gun around with you.
Triple K makes leather holsters to fit less common models. They are more hunting or hiking style, but well made. Sportsmans Guide carries them and I think they have a website.
Last but not least is the custom maker. I don't have specific recommendations but if you are truly dedicated to your Witness you might ask around. I have no idea how much this would cost. However, you would probably get a great personalized rig to be proud of.
