The Windows Phone 7 SDK beta includes a set of the basic core icons. They live in the
C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.0\Icons
C:\Program Files\Microsoft SDKs\Windows Phone\v7.0\Icons (x64)
They also provide a vector based version of most of then in the
\vector folder, which is super useful if you are going to go about creating your own variants. The problem is they are provided in Adobe Illustrator’s
.ai format. The good news is that if you have a copy of Expression Blend you can import them straight in.
If you don’t have Expression Blend you could use the open source Inkscape. I have used it before, it’s a bit clunky but it gets the job done.
When creating custom Application Bar icons remember that you do not need to create the white circle around the icon. That will be added by the application bar itself. The Design Templates for Windows Phone 7 includes some guidance on Application Bar button design.
Here is a vector graphics version this image (as a Expression Design
.design file) which you use as a starting point to create your own icons.