Hard rubber stocks often come up for sale on ebay and on the standard online gun sites like Gunbroker, GunsAmerica, Gunsinternational, etc. Make sure you get the right size stocks, since S&W made several different frame sizes. The stocks that fit a 38 Safety, if that is the model you have, are the same size as any of the pre-war I frame (32 Hand Ejector) round-butt stocks. You can also post a WTB add on this Forum.
Check this out for measurement the measurements you need.
http://smith-wessonforum.com/blog.php?b=111