Tumorhead is located in Lectra City on the planet Promethea. You want to make your way to the Underpass area. Go past the parking garage and go down to the stairs. You will find Tumorhead underneath the city in one of the corridors in the underground subway.
You can farm Tumorhead for these legendary weapons as it is the dedicated loot source for these items. Tumorhead has a 15% legendary drop chance.