I second the rebuild idea. The Mercedes service manual for the 616 engine shows how to rebuild it.
I think the water pump in the G-Wagen may also have been used in the 309d etc commercial vans sold in the rest of the world. I bought the one for my 307d at Europa. I could be the one person they sold a pump to! In my case, the pump was very unusual in that the fan clutch attached to the pump with one large central bolt instead of the usual 4 bolts.
You could also try Euro-Tech which has access to the European parts list. You might also try buying directly from a European parts supplier, perhaps in the UK. There are a number of suppliers in the UK on the web.

The other option is to take the Cuban mechanic approach. Look closely at your pump and see if one from a 300d might fit. On my 307d, if Ihad been able to replace the fan clutch and fan (from a junkyard?) I could have substituted a pump from an auto engine.
From looking at my manual, some pumps have different sized bearings, but I'm betting an auto engine pump might work just fine.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13