Have a new Mac
If you are still within your 15 day return period. Call Apple they will sent out iDVD at no charge. (There are no realistic substitutes.)
Nicely explain that you may return the computer without iDVD. That usually does the trick. You MUST speak to a senior advisor to get a free copy. The first tier people cannot do it.
If that doesn't work you can purchase iLife 11 (includes iDVD) on disk.
IDVD is a wonderful piece of software and well worth the low cost of $40.
I've had my Mac since July. Just now getting around to burning some DVD's. I did call and they state that the iDVD discs that they had available to mail out are no longer available. Bummer. My uncle also suggested Roxio Toast 11, but I'm concerned about compatibility issues with ML 10.8. Any thoughts?