Hard to find any 813's in the USA. There's a guy in the Pacific NW who specializes in Mercedes vans, trucks and Unimogs. He may be your best bet in the USA. Have you tried European Ebay sites? You also might post the question over on Expedition Portal website in their Mercedes Truck and Unimog forum.

EUROTECH SERVICES INTERNATIONAL, UNIMOG

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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