On this parts fiche the Nut,cap,6mm is the most specified component.
The most reasonably priced component used on the Gl1500c Valkyrie 1998 (w) Canada / Rbm Exhaust Muffler is the Washer,plain, 6mm.
Some items from this specific schematic: Nut,fla.cap 8mm, Tail Piece,r, Muff.comp.exh.r, Washer,plain, 6mm.