As many of you are aware, there is a standard process notation standard out there called Business Process Modeling Notation (BPMN). If you are looking to use this notation in Visio, you have several options. The most straightforward is to just use "Search for Shapes" (at the top of the shapes window) by typing "BPMN" into the search box. This will download a collection of the shapes to your machine and you can just start using them. One note is that the resulting shapes are based on a version of the spec right before the final one. This feature in general is something to keep in mind when you need a set of shapes that we don't ship in the box.
If you prefer to have your shapes set up in an organized stencil and templates, you can look at a couple of available stencils:
Finally, if you want to make sure that your BPMN is fully validated (for example, so it can be exported into a BPM system) you might want to try some of our partners. I'm very familiar with itp Commerce, but there are 192 partners listed on the Visio Solution Partner page, many of whom are in the Business Process Management industry.