Joined: Feb 2005
Big O is set in a largely 1920-30 American setting. Suits and the lot included.
FMA: The Conqueror of Shambala (While being quite terrible) do feature a little bit of a European pre WWII setting.
Legend of the Galactic Heroes draws a lot from historical 18th century settings, also in design, at least for the Empire's part. I did miss the Top hats though.
Princess Tutu should also fit somewhat, at least when they're out of the titular costumes.
Rose of Versailles is set in the 18th century.
Bradherley's Coach is right on the mark. Quite grim and disturbing though.
Hellsing has a couple of characters in suits and at least one with the full package of top hat as well, but they're not quite the main focus after all.
Shin Angyo Onshi takes place in a 1800-ish setting. Not much of suits and top hats at first, but once the story takes a tour over to the "west" somewhere, it pops up quite a bit.
Along with Emma, Shirley is another 1800 centric manga.
Eigi man ek þá lǫg jómsvikinga ef ek kviði við bana eða mæla ek æðruorð. Eitt sinn skal hverr deyja