I'm looking at the same issue right now and found the following sources for both remanufactured heads and rebuilding services for Mercedes heads.
http://www.mercedesengines.net/categories.asp?cat=17
http://www.autoshoppingcenter.com/rebuiltengines/mercedeshead.html
http://www.ronscylinderheads.com/mercedes.html
Cost runs about $940 + core and shipping for a remanufactured exchange unit, or $490 for basic head inspection, cleaning, and a valve job on your head. (Extra charges for a really screwed up head with multiple bent or burnt valves, etc..)
Shipping is about $80 round trip coast to coast for ground shipment.