Holy sword of the Order can be found on on the body of Patrick de Weyze after a battle during one path of a Chapter III sidequest. It can also be found on the body of Siegfried after you kill him in the streets of the Temple Quarter in the Epilogue. Can also be found in barracks near order smith.
Also it can be found in the wardrobe in Siegfried's quarters. Looks like a broader two-handed longsword with a gold/jeweled/ornate hilt.