Hi
You can get it here -
Comand Online Ltd :: Mercedes Phone Retrofit Kits
you need
a) PHONE-MOST
b) W203/W209 centre console parts if you want the phone connector in the centre armrest. But you can just put it in the main glovebox with a bluetooth connection as you won't need regular access to it
And then from here, either a MB-2 which is a generic bluetooth cradle, or HFP (which is the genuine Mercedes equivalent) or the iPhone 3G cradle, which is a bluetooth cradle but also charges phone and can be connected to an iPod kit (with an extra cable)
Comand Online Ltd :: Phone Cradles & Adapters
A competant audio shop can fit it, or if near woking in UK talk to
sales@comandeurope.com, and I know of lots of DIY'ers that have installed it.
Richard