If the Mac in your signature is what you desire to use 10.5 with, you can't do so, it is too new.
If PowerPC application compatibility is what you desire with that Mac, look at this solution:
10.5 only was distributed by CD and updates to 10.5 from free online downloads. Why do you want 10.5, and what do you desire to put it on? Is the person sending 10.5 using 10.5 for anything? If he is, he can't sell the drive.
I have often used the latest free version of Carbon Copy Cloner to back up whole OSs, applications and user files etc to a disk image. Whilst I have backed up earlier OSs than the boot volume they have not been widely different and I don't know what the limitations are except that they must be the same hardware - ie use Intel to backup Intel and PPC to backup PPC.