You need to first get Sideshow Bob. You get him upon completing Flaw and Order Pt. 3, which requires you to arrest Sideshow Bob.
Thing is; there is a measly 10 percent chance for him to randomly spawn in your town, of which there will be a notification of him in your town.
(Hey, it could be worse. We could be playing the game pre-Terwilligers event, and pray to the RNGod to bless us with the 1 PERCENT chance for him to spawn so we can arrest him.)
Showbiz Bob is a costume as well I believe. You would need both SB and the costume.