Holsters made for S&W K frames also fit Security Sixes. Many are marked for both frames but even among those that are not marked I have not found an exception. Fortunately these are the most common leather belt holsters on the used market. There are tons of old police 4" K frame holsters and a lot of used concealment holster at gun show and stores.
It's always best to take your revolver shopping and trial fit to be sure. Occasionally a leather holster will be a sloppy fit because a prior owner used to for the next larger frame size. I did that once because I couldn't find an Colt Officer's Model holster locally. K frame leather will stretch to fit L frames and older Colts.