The final Memento quest in Bloodstained: Ritual of the Night tasks you with finding orichalcum. As simple as fetching an item might be, this particular one isn't as readily available as you might be inclined to believe.
Luckily, we found one source that drops orichalcum, and can let you know where to look for it in Bloodstained: Ritual of the Night.
Bloodstained Ritual of the Night Orichalcum
Finding the orichalcum in Bloodstained: Ritual of the Night is tricky mainly because of its low drop rate. There's a 1% chance that it drops with every kill.
You'll want to head over to the Glacial Tomb. There you'll find the G Axe Outsider enemies – large axe-throwing demons. They're the one source of orichalcum we found so far, so you're going to have to kill as many as you can. It's worth noting that the Zangetsuto blade is able to deflect their ranged axe attacks which might make fights a tad easier.
It's possible there are other sources of orichalcum in Bloodstained: Ritual of the Night, however, they've eluded us thus far. Nonetheless, with enough patience, you should eventually get the item from the G Axe Outsiders.