The are a service item and will be available from a stihl dealer
Most Stihl shafts are available as aftermarket parts as well.
Stihl guard their information in a mannar that is directly descended from the SS so you won't find much info on the web.
So you will have to ring your local independant repair man who might have access to the aftermarket suppliers or you friendly Stihl dealer.
If you friendly Stihl dealer gives you the run around then pop off a request to Stihl direct.
They take customer service very seriously which is why they keep everything as a big secret.
I can get around 40 aftermarket Stihl shafts down here, but yours is not one that is listed in my catalogues.