Haow can i get o-parts(purple) upgrades except xsotan

Im another beginner..

I got 7 xsotan upgrades except second xsotan that can be made from compose of 3 o-parts upgrades..

But i cannot find o-parts upgrade. Now i repeatably fight with brotherhood (which can give me a o-parts upgrade...in my opinion...because he is a boss!), but i didnt get one.

If there is certain npc that gives me a o-parts upgrade?

I've stuck here and cannot go forward.

Thanks