Well, without the original CD-Key, the only way to get another legitimate one is to re-purchase the game. You can do it online through Digital River and I think Direct2Drive for digital download only purchases (which will give you the right to download a copy of the files, and the CD-Key, which you should be able to use with your Retail CD/DVD copy as well -- just not both at the same time on the same machine and you cant't use both copies, using the same CD-Key to play online at the same time.)
Other than that, I'd scour the likely places where that manual might be. The CD-Key is on the inside front cover. And this is why I simply don't loan out my games to anyone without me going along with it, installing it, then taking my stuff home. If they like it that much, they can go buy their own dang copy. They're not losing my stuff. I'd already lost too many books and albums to my friends' indiscretion to make that mistake by the time I got a computer.
dunniteowl