Here is a link to the S&W replacement sight on Midway:
Smith & Wesson Rear Sight Blade Kit K, L, N-Frame with .146" White Outline Blade, Windage Nut, Screw and Instructions - MidwayUSA

I really like that Millet sight, but they seem to be hard to find. Midwest Hunters Supply looks like they might be stocking some, but I couldn't quite figure out from their website.