The most obvious is to print-screen. Then open iTunes in your browser.
If you are using Safari go to Develop > Show Page Resources.
Go to images and find the one that resembles a frame. You can then easily edit the screenshot in photoshop by selecting the frame and cut out the selection from the screenshot. You have to play with positioning a bit to get precise results and then add shadow.
Another way is to download the App > double tap > show in finder > find the App > change extension to zip > unpack > open > show package contest > find icon within.
The only problem is that some iOS Apps don't contain icons in larger sizes.
The best solution however is to contact the devs directly. In most cases, these guys are incredibly helpful human beings, so don't forget to send an honest appreciation, too.
Also, the chances are someone else has already done all the above, posted it on their website and you can easily get to it through image-search.