Icons for the windows 8 app bar
You get around 200 icons with the developer tools, use them, Microsoft has done that job for you.
If you don’t find the right button for your app you can use one of my favorite apps Metro Studio 2 from Syncfusion. It’s a free app with over 2500 “metro inspired” icons. You can even send in a request for a new icon.
Metro Studio 2 allows you to do some easy editing like change the size, color, background, padding, rotate etc.
Other resources worth mentioning is http://www.ModernUIIcons.com (former templarian I think) They offer 1100 icons in expression design format, svg and xaml.
and http://www.geekchamp.com/icon-explorer/ that has over 1500 icons in png, xaml and html.
There is a Windows 8 app called “xaml icons” that offers over 10000 vector icons. I have yet to try it but 10000 sure is a lot of icons!