Broken armour can only be found by digging in the Digsite, pickpocketing H.A.M. Members and doing the Fishing Trawler minigame. It can be repaired at an armour stand in the workshop of a player-owned house. It will then become a platelegs or plateskirt of random metal, from bronze to mithril. Also, there is a chance of breaking it beyond repair.
Archaeological expert Terrance Balando at the Exam Centre in Misthalin says, "...this is the armour of a legendary warrior".
Note that you need level 55 Construction to build an armour stand. However, you can use an armour stand built by a friend, in their POH.