For this you will need the iLife ’09 or ’11 installation DVD: Purchase iLife – iWeb
When you get to Installation Type: Click on the customize button at the bottom
Here the installation will allow you to choose which applications to install.
Select iWeb & click install. (iDVD can be installed at this point if you want. In that case you would select iWeb & iDVD then click install)
After the install is complete run “Software Update” from the apple menu or the app store. There will be some updates for iWeb/iDVD to allow them to work with the other iLife apps
iWeb is available on Amazon as part of the iLife ’09 or ’11 box set (iLife ’09 is a little cheaper and contains the same version of iWeb).