2 options:
1: Blizzard will replace the damaged CD at a (fairly) low cost.
2: Borrow a CD, use Alcohol120% or Blindwrite to make an image of the CD on your hard-drive, then use Daemon-tools to mount this image on a "virtual CD drive" (I believe I've read that Alcohol120% can do both jobs); from then on the game will believe you have a new CD drive, and it has the disk mounted in it. Your real CD stays in a locker safe from vandals, thieves, children, pets, pizza, and coffee-mugs.
The third option of buying a new one goes without saying (so why AM I saying?)