iChat Download For OS X 10.6.8
The solution is reported on this discussion on the Apple Support Communities. 

They report several ways to do that.
The easiest way seems to be:


  Insert your Tiger/Leopard/Snow Leopard DVD into your drive. Once the
  DVD loads, open the "Optional Installs" folder and then the .mpkg of
  the same name. This will launch an Installer. Click through until you
  get the option of choosing what you'd like to install. (this is after
  the Licence Agreement part)
  
  Open the "Applications" by using the tick box and triangle for the
  drop down menu in the panel of software to install, simply choose
  iChat (only) and let the installer do the work.