There should be no cost at all for the demo disc. As for the network disc, you may have to look to eBay to get one if not available from Sony themselves. Again, you will have to talk to Sony about it at the tech support number found in the manual.
As I mentioned in your other thread, the sites that are offering free PlayStation 2 games are doing so without consent from Sony and are 100% illegal. I ask you to kindly not bring up the topic of downloading the games again.
There are no demos that you can download, either, but some games do have demo versions available on disc and can be found in stores as a promotional item or, in some cases, from the game maker themselves through a sign up form on their web site.