Boot Camp Assistant Missing?
You can use the Recovery partition to reinstall OS X. This will keep your user account but reset all system files.

To reinstall OS X, boot with ⌘R and select the Reinstall OS X option, then follow the instructions.

Additionally, this file is normally backed up to the local time machine store, so try entering Time Machine even if you don't have your drive connected to see about restoring that file. 

Worst case, have some trusted people MD5 check sum the app on 10.8.5 if you want to risk downloading it from a stranger instead of from a friend's backup or Mac.