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 ...