I assume you mean the Tele Atlas Navigation CD ROM.
If you have COMAND 2.0 or 2.5 you need something called a 'DX' disc. These are very easy to goet hold of and I found the easiest and best source was 'www.navshop.co.uk'(thisa site run by Tele Atlas). The newer navigation units fitted to the latest E class and facelifted C & S Class are DVD based.
If you don't find what you want there you can always try ebay, lots of people appear to sell their older Navigation CD's on there. I have seen some 2003/2004 navigation discs on there but they do seem to fetch fairly high prices (£55 - 75 for a used CD!)
