duke′s guard armour you get in the legion as knight of the garland,actually after the first quest to finally be knight of the garland(it is two quests),or the very similar looking ordinary imperial silver cuirass,think the smith in caldera sells at least one of them iirc.Luckily it is a nice silver helm to go with it.
to bad it isn′t a full set of those,ordinary,so you need something other for the rest besides the cuirass and helm,imperial legion pauldrons,greaves and boots for example,adamantium are slightly out of touch with its more greyish appearence but could work in a pinch.Or you cover some of it up with a skirt "redoran-style"