I don't know of a source that has only the shapes in a stencil, but there
are a couple of
third-party VSM add-on solutions for Visio that include them:
Also, there is a download of VSM symbols available at the link below. They
can be used in Visio but they are BMPs and WMFs, not Visio shapes. http://www.strategosinc.com/dl0_vsm.htm
I have not found one either, but looking. I looked on the internet but
found nothing.
I am sure you know what you're doing, as vision would only be helpful after
the fact. VSM is best done with a pencil on a large piece of poster paper
at first.