Breakers are your best bet, although you may be only able to get one at a time & they may have broken mounts- fortunately its possible to get replacement mounts from MB.
They are likley to be £200 upwards from breakers
These:
http://www.cksperformance.com/mercedes/index.asp?SID=31 do brand new units for £1000/ pair.
Europarts or Mercman charge somewhere between the two
I will be listing my facelift halogens on ebay soon