The term is called software piracy getting it for free and the reason its called piracy is because its illegal.
Plus that's if you can even find 10.4. No Apple authorized shop in the world is going to sell you 10.4 because they can't even get the disks from Apple anymore. If you shop online you may be able to find 10.4, but for what they are asking it may just be cheaper to get 10.5, if your computer will even support it.