I wouldn't get the Serpa (unless its their Serpa Sportster) with a gun with no safety. The button release is right by the trigger and people have shot themselves drawing. Don't get me wrong I've got one and use it, but it's for a safety-equipped P-95. Also to be frank, concealing a kydex belt holster ain't exactly easy.

Look for a "pancake" style leather holster.

Densantis makes them in both thumbreak and open-top models:

DeSantis Thumb Break Scabbard Belt Holster Right Hand S&W SD9 SD40

DeSantis Speed Scabbard Belt Holster Right Hand S&W SD 9mm 40 S&W