Sprocket guards are readly available from many companies. LSR, GT Thunder, and many others. 450R, 400EX, 300EX, and, 250R all the same rotor patterns so even if a company does not list one for an R another application will fit. Dual sprocket guard are much more costly because one of the guards has a machined lip.
Bling Star was the only company that made a off the shelf bolt on rotor guard. The way it was bolted on transferred the impact force to other parts. looks super trick but haven't heardmany good reviews of it.