See if you have confident doing this simple DIY project. I saw one selling a claimed to be genuine part, $8.99 (plus shipping) on the Ebay from a USA seller.
Instruction link : http://www.focusingscreen.com/work/7den.htm
I have done easier replacement of focusing screen on my older 40D and 600D and have not done the finder focus adjustment as indicated in the 5D maintenance manual. Nothing went wrong with focusing and metering or may be I'm just lucky. For very precision adjustment, Canon Service may be the way to go. But for $10 to $15, the risk seems very low.
Disclaimer : I have absolutely no connection with the parts provider as advertised in the link I posted above. If you considered to DIY, you may buy from Canon or the other online sources.