Do you have your original OS X discs that you used before Tiger? I think you need to put that in and pick the 'optional installs' file on it. It's possible it's on another disc that came with your Mac, but check the original OS X disc first.
iLife only comes in two ways:
1) As an all new version in its own box ($79) each year or
2) It comes with a new Mac
It does not come with seperate, new versions of OS X. So when you bought Tiger it was not on that disc. I'm guessing you erased the drive and did a clean instal? That wiped it out.
But you should be able to find it on one of the discs that came with your Mac. (Either that or buy the new version that just came out.)