If you feel that the stencils that come with OmniGraffle are not adequate for your use, there are a few sources where you can get more stencils.
On the Omni Group website you will find a few selected stencils you can download. There are also some links to external stencil makers. Check out http://www.omnigroup.com/products/omnigraffle/extras/.
Graffletopia is the premier website for OmniGraffle stencils.
Graffletopia is a community effort open to anyone who has created a stencil. These stencils need not be very advanced (i.e. there is a stencil with just a mouse pointer), but a lot of the stencils available from the website can be quite complex.
As of writing this book, there are more than 400 different stencils, grouped in 12 categories. New stencils are added every week.
The good people at Omni Group (the makers of OmniGraffle) have recognized this valuable source making it possible to search and import stencils from Graffletopia directly...