If you have to purchase an iLife disc remember that iLife 11 only provides iDVD 5-7 themes. The Software Update no longer installs the earlier themes nor do any of the iDVD 7 updaters available from the Apple Downloads website contain them.
Currently the only sure fire way to get all themes is to start with the iLife 09 disc:
This shows the iDVD contents in the iLife 09 disc via Pacifist:
You then can upgrade from iDVD 7.0.3 to iDVD 7.1.2 via the updaters at the Apple Downloads webpage.