I want to make some small modifications to the default control template of some controls in my windows phone 8 app. The problem is that I can\'t find these default templates
You can find the file containing all the default control templates here:
C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Design\System.Windows.xaml
Edit: also, the Windows Store Dev Center has a lot of them listed if you search for "styles and templates"
I'm not aware of such a listing for WP8, however, you can use Blend or Visual Studio to edit a copy of a control's current template by right clicking on the control and then selecting Edit Style, Edit Template or Edit Additional Templates as appropriate. In the submenu, you can select Edit a Copy... which will present a window that allows you to create a copy of the default (or current template/stlye if you have already modified it) and customize it.