Where can I find decent visio templates/diagrams for software architecture?

For SOA system architecture, I use the SOACP Visio stencil. It provides the symbols that are used in Thomas Erl's SOA book series.

I use the Visio Network and Database stencils to model most other requirements.

Beautiful set of stencils from Microsoft here.

Here is a link to a Visio Stencil and Template for UML 2.0.