As Sam mentioned, the S&W Model 14. They are not all that easy to find (the single action only version). I looked for one for awhile before I finally gave up and bought the standard double action model. I then tryed to send it to the S&W Performance Center to have it modified to SA-only but they no longer offer that service. One further note, if you do want to buy one for collector purposes, find one that came from the factory in that mode; one that was not later modified to SA-only. This would require some research.
Why ? Why not ? It is something a little unusual, something fairly unique. If I want to shoot double action I can use any one of my few dozen double action revolvers. My model 14 is an 8 3/8" model with a set of Randall Fung grips on it. It is fired exclusively with wadcutters from the classic bullseye stance. It is not really something that lends itself to double action shooting. The gun world is not all about self defense. Many people enjoy collecting, target shooting, and some people just like to own a particular gun because it appeals to them. I certainly own a number of single action Rugers, why not a S&W ?