Hana tour is one of the major tour companies in Korea. (providing China, Japan, other Asian country tour from Korea for Korean people)
I found it offers a tour service to Non-korean people.
not sure about the quality.
AKPM10 Panmunjeom Tour
http://www.hanatour.com/eng/english/tours/AKPM10.htm
10:30-16:30 (Actual tour duration)
AKPM09 DMZ & 3rd Tunnel Tour
http://www.hanatour.com/eng/english/tours/AKPM09.htm
09:00-17:00 (Actual tour duration)