You might search the auction sites like eBay and Gunbroker, sometimes you will find a 5906 slide with the factory adjustable sights. Otherwise, you will need to look for an LPA adjustable rear sight or, if you can find one, a Millet adjustable rear sight.
If you go for an aftermarket adjustable sight, remember there were two very different rear sight dovetails on the S&W 3rd gen pistols. Early production 3rd gen pistols, the ones with hooked trigger guard, usually have the 2nd gen style rear sight dovetail. Later production 3rd gen pistols have the slide cut for Novak rear sights.