I needed an ivory set into a brass sight several years ago. Ended up taking rod, drilling and tapping it to hold the sight and then turned it down on my metal lathe. Took a scrap of ivory and glued in onto the sight and turned it to shape. Finished with a little tea to "age" the ivory. Plain steel or brass sights are easy to come by. Perhaps Brownells has one in the size you are looking for. Any home hobby lathe operator could do what you are wanting in half an hour.