Not sure what she would have for Selective Mutism, but Jill Kuzma has some good ideas.http://jillkuzma.wordpress.com/
She bases some of it on the work of Michelle Garcia Winner, who is awesome for social thinking. A lot of what she does was originated for people with high functioning autism or Asperger's, but can be applied where needed. Hers isn't a site with ideas so much as with her books (Superflex is wonderful!).www.socialthinking.com/
Also, social stories, developed by Carol Gray are great tools for lots of kids. You can read through her website for more about social stories.www.thegraycenter.org/
You could probably google "social story" and a keyword that you're working on to see if there are any posted online. If not, they are not too hard to write if you have a basic structure.
Hope that you get some ideas. Good luck!