In that case just purchase a retail copy of iLife and install the iDVD app. Once installed, update it to the latest version 7.1.2:
http://support.apple.com/kb/DL1414
Apple no longer sells iLife but it is readily available from sources such as Amazon. I have personally confirmed that iLife 11 and iLife 09 both contain the same version of iDVD, and have heard that iLife 08 does as well. The price is still reasonable. There is no DVD authoring product worthy of comparison to iDVD.
If you have the iDVD app on another Mac you can simply copy the app from the old Mac. As long as it is not a very old PowerPC version it will work.