They show up on eBay. Be careful when you get one as they are a very delicate design with a real weak windage spring. If you are not too concerned with originality, Redfield made a much better site that can be attached to the rear of the receiver, but you will have to drill two holes. I have also seen early 19s with receiver sights made by Savage/Stevens for the 416. But again, you will have to drill a couple holes. I think I may have one of those if that is somethig you'd be interested in.
