While I was preparing my first app Short (which will launch on 16.02.2015) for the App Store I made these iPad App Store Assets in Sketch and thought it could be helpful for you, too. It includes multiple layers where you can choose between a space grey, white or gold iPad, a colored background or an image as a background in landscape and portrait.
It also includes a shared style for the backgrounds, so you just change the background once and every other artboard gets updated automatically.
Feel free to use it for your projects.