Hi all,
This is an update. I have found Icomoon to be a good place for icons for the W8 themes. Icomoon consists of several different icon collections gathered together in a custom app. You can use the Icomoon app to resize icons, recolor icons etc. The optimal size for e.g. tab icons in the W8 themes is 12x12 and with the Icomoon app it´s easy to resize icons to that size.
Icomoon: http://icomoon.io/
PS: The actual icons used in the W8 themes can be found in one of the collections in the Icomoon app ("Iconic" collection).
