Call S&W. The front sights are like $1.50 each or so. The rear sight with the screw is about $30. I damaged a pair once by dropping the gun. SO, I keep an extra set of sights.
Get the #3 and the #4 front sights (or just buy the whole set of 4, just to have).
The #4 is what comes on the gun - my SW99 shot low and I had to put the #3 on it.
Also, make sure to buy the set screw for the front sight.