I have also replaced the focusing screen of my Df with a K-3 from focusingscreen.com. I have found them reliable, with a fast delivery. I initially intended to replace the screen myself, based on my experience with Canon bodies (EoS 1V and 6D), but I found it much more difficult. Before breaking something I went to a small photo repair shop in Paris who services my film cameras, and they were happy to do it for a small fee.
As regards usage, I prefer to use manual focus anyway, so a split-screen really makes a difference to me.