Where can I find screencapture to replace mine that is broken?
If you had 10.7 or a newer version of OS X, you could download an OS X installer from the App Store and use Pacifist to extract the screencapture binary from the installer. I'm not sure if something similar is possible with a 10.5 installation DVD.

Alternatively, if you upgrade to a newer version of OS X, it should also replace the screencapture binary with a new version.