The decorative magic robe top can be bought from the Castle Wars lobby for 40 Castle Wars tickets. The item set can be purchased from Lanthus. It requires level 20 Magic to wear.
Upon release, the Decorative magic robe top required 80 Castle wars tickets to purchase. This was reduced to 40 in an update on 11 May 2017. If a player bought one before this date, they can speak to Lanthus, found in the Castle Wars lobby, to reclaim 40 tickets.