Please help,

I have recently purchased a 2005 touareg which had a standard cd head unit. I quickly decided to upgrade the head unit to a cd rom navigation unit. The problem is that i cant find any cd navigation discs for it. I have purchased several online and the head unit just says disc not compatible. Before the disc is inserted the screen says insert radio navigation s2 cd .
Can anyone help and tell me what disc i need or is there anyone out there who can supply me with one ?