cd changer (factory)
e-bay or dealor.
You might also try junkyards or MB parts dealers from online -- lots of guys specialize in stuff like this.
I got mine from AutoToys.com. They sell an OEM Alpine 6-disc changer w/ a converter module to allow you play MP3s with the original BOSE head unit. I've had mine almost 5 years on my '96 S420 and love it. I just noticed you have a '99. They say it only works '95-'98, but it's worth e-mailing them and asking if it will work for yours. Here is a link: http://www.autotoys.com/x/product.php?productid=1407
Looking for a Used S-Class in your area?
CarGurus has 5,975 nationwide S-Class listings starting at $2,500.
Search Mercedes-Benz S-Class Questions
Mercedes-Benz S-Class Experts