Apple no longer distributes iDVD.
1a) Buy an old iLife from a place such as e-bay or Amazon (https://discussions.apple.com/message/18750180 or http://www.megamacs.com/index.php?action=frameview&id=404733009&D=1). Make sure it is a version that has iDVD. Do not buy it from Apple since their version no longer has iDVD.
1b) Depending upon which computers and discs you have you may be able to do a custom installation of the software using old system installer discs that had iLife bundled.
2) Try a third party application (none have the features of iDVD though).
Burn (free) - http://burn-osx.sourceforge.net/Pages/English/home.html - includes basic DVD authoring but you will definitely have to read the documentation.
Easily Burn Movies to a DVD without Apple's iDVD. - http://www.creativetechs.com/iq/easily_burn_movies_to_a_dvd_without_apples_idvd. html - directions for using Burn to make a TV DVD. Note, Burn also has a separate application which lets you do more fancy menus.
Commentary on DVDs by Klaus1 - https://discussions.apple.com/message/18338996