Last year I ordered the full rear sight assembly for the 686 direct from Smith & Wesson in the US. They have a nice lady that works in their parts order department, who really knows her stuff. She will first try to determine if you need the old assembly, or the new assembly. I believe she set me up with the new assembly, but with an adapter to put on the old guns.
I ordered some other miscellaneous screws for a couple of pistols, and the parts for the revolver; I think it cost, with shipping, under $100.00. Doesn't hurt to call.