While plastic chain guards like the one the OP has aren't generally offered in the after-market, there are a number of other options which serve the same purpose.
If the crank has a standard 110mm or other common BCD he might buy a bashguard, or chain keeper ring (like for cyclocross) which will serve the same purpose.
Otherwise there's the universal (two sizes)
Chain Disc sold by the folks at KoolStop.