Typically, unless VBA involved, the Visio community provides stencils for free.
Google visio stencils
If you are a looking for a repository of stencils...no single site covers all
visiocafe.com has lots of product stencils
MS has stencils for MS products as well as education stencils
VMware has stencils
RFcafe has stencils for hardcore communication theory
VREEAM
etc
all are free