I made a short video below showing the process of changing the focusing screen in the Mark III with an Eg-s Focusing screen.
http://www.amazon.com/Canon-Eg-S-Precision-Focusing-5D-Mark/dp/B001KBZ3S2
You can purchase specialized, third-party focusing screens that are made-to-order, from only a few online retailers. But prices for those screens are $90 +
http://www.focusingscreen.com/index.php?cPath=21_134
Hope this helps! Let me know if you have any questions!