Upgrade disc?

I don't think Apple is even offering it.

http://en.wikipedia.org/wiki/Mac_OS_X_v10.5#Version_history

 

I wish it was, but Apple only puts out "reference" releases every once in a while. For 10.5 it was: 10.5.0, 10.5.4, and 10.5.6.

There are two routes to go with that information: you can either hack the Apple installer to add in the latest updates (not too difficult), or you can go with something like InstaDMG which produces ASR-scanned images (this route is good for when you can wipe the computer, like in a lab).