He likely did not use authentic clothing since there isn’t enough of that to be had (let alone the right size for the actors, and would be let out the musuem to be worn. I have little doubt that he got exact patterns from actual costumes.
That said, BL is very, very accurate. ALthough if you look very closely you might notice a few of the Prussian soldiers are carrying Mausers.
My nomination for “atmosphere accuray” would be the Richard Lester’s Three Musketeers and Four Musketeers. With a handful of exceptions (Like Milady’s 19th century pistol and Athos’ jack-in-the-box wind up wheellock) it really gets the feel of that era.