Ec-A Focusing Screen from focusingscreen.com which is the best place to get them from. Made from Canon screens to fit your Nikon Df. Pretty awesome stuff! Look at the included pictures from their site to see what's it gonna look like. Helps tremendously to focus manual lenses (and why get the Df otherwise?)
Now, I scratched it while installing. The included tool slipped. The scratch is on the very top but is still visible. I stopped noticing it after a week of use but YMMV.
Paid over $100 for it and selling for half the price because of the scratch. Will come with the tools and everything you need to install the screen - very straight forward. Here's the istruction if you need to see the complexity of the task: http://www.focusingscreen.com/work/d800en.htm
This is the product page: https://www.focusingscreen.com/product_info.php?cPath=22_139&products_id=1240&osCsid=27613a5d271a8527904f9170ac17760d
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