If you just need icons for the application bar buttons, then start with the ones that come with the SDK. You'll find them at "C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Icons\Dark\" and these are also the ones that Blend can add for you.
If you need more then there are a numberof free resources available:
Metro Studio 2 (By SyncFusion)It's a downloadable app that comes with 1700 FREE icon templates that can be sized and modified as required. You can see all the icons at http://www.syncfusion.com/downloads/IconPackagePopup/Application
There are also other sources and you could always make your own or commission an icon if you needed something special.
As an extrs tip, it's usually best, as far as possible, to get all your icons for an app from the same source. This helps make it look like they all belong together. If you have small/subtle differences between icons (e.g. differing line weights or the radius on rounded corners) then some people will spot this and will think it looks bad. Remember, the little details matter.
Are you a Windows Phone, Nokia-X (Android) or Asha developer? If so, you could be getting rewards for the apps you build and the success they achieve by joining the DVLUP program.