It's a long shot but no harm in calling Apple to see if they can help. (They surely have plenty of Motion content disks so that they coulld easily copy one for you.)
This is reminder of why it's a good idea to make back-up img files…guards against lost and damaged disks and much faster re-installs. Good luck.
Maybe you know someone who has a copy of the disc your missing. Or, go to a Final Cut or Mac meeting in your area and see if someone can help you out.
If and when you get this sorted out:
Another good idea is to make image discs of all the discs (FCS) and store the image discs (47.79 gigs) on a separate drive or even a thumb drive. And put the FCS discs (box) in a safe place. Along with the serial number... (I have the serial number stored on the cloud. Cut and paste... could never get the serial number right on the first try .)
An added plus... the image discs will install FCS faster compared to using the DVDs. (In about 35 or so minutes.) Each image disc will load when needed. You don't have to baby-sit the installation.