You can farm Katagawa Jr. for these legendary weapons as it is the dedicated loot source for these items. Katagawa Jr. has a 15% legendary drop chance.
Katagawa Jr. Guide
The Katagawa Jr. can be very short and easy. When you jump into the arena, immidiatly start shooting at Katagawa Jr. with a shock weapon. This will insta-kill him for some reason.
The normal Katagawa Jr. fight consists out of 2 elements. One where he will be on the floor and attack you with his sword. In the other, he will hide on one of the towers and shoot you with his sniper rifle. During this phase, there will be several digi-clones in the arena.
The digi-clones die immediately when you hit them, the real Katagawa Jr. is the only one that takes damage. You can shoot the generators in the arena to quickly dispose of the clones or severely damage Katagawa Jr.