As the title says, I have searched and for the life of me nothing says WHERE I can find it.

It drops on weapons/units from Deus Esca (all forms).

It also drops from Esca Falz Mother too.

In addition, both NA Arks Visiphone and JP Swiki (Dekor Maryuz / Esca Ragnez) show the soul drops from Dekor Maryuz and Esca Ragnez too.

Just met Dekor Maryuz in Tokyo Exploration XH and it dropped Escarde Soul on a unit