A few years back, when I had my 2001 S203 C270, I needed the bush replacing - and had to buy a complete ARB because the bush is moulded to the ARB and could not be replaced on its own. The new ARB was not too expensive so I did not investigate further, especially as I was having various other bits of the front suspension replaced at the same time.
Worth a look at partsreunited.co.uk, which is where I found the front suspension parts I needed (at a good price, from MB Manchester or Birmingham, can't remember which).