The rear sight blade kit is $11 and it used to be on the Smith & Wesson website. It is just hard to find since they redesigned the site
Different rear blades go with different length barrels
Just measure the rear sight blade that you have and order the same height from Smith & Wesson parts
If you do not own a micrometer or caliper to measure with then call Smith & Wesson give them your serial number and tell them what you need
You will have it all resolved in one phone call