Looks like the main ones that organize DMZ/JSA tours are
1) USO from Camp Kim
2) Panmunjom Tour as mentioned in post #2
3) International Cultural Service Club
4) Tour DMZ from Lotte Hotel
5) Cosmojin Travel - 87,000 won
Both Panmunjom Tour
shinjukuflyer mentioned and Tour DMZ
http://tourdmz.com/english/main.php seem to both depart at Lotte Hotel 6th floor / both "includes" lunch. Maybe they are basically the same tour?