Ros Arago IV

[alpine ferry]

It's a world drop, it should drop randomly for vendor engrams (any except playlist vendors like Shax or drifter).

[last elbow]

Gather as many gunsmith engrams and each week all the world drop weapons are on rotation. See which day he sells it and buy it

Might be able to get it from expert lost sectors as well if it’s in the loot pool table