I would have guessed that the caps are a standard Suzuki part common on many alloy wheels.
Might be worth considering aftermarket nuts since the plating doesn't seem brilliant on the standard parts. Custom goodies type retailers ought to stock something suitable, need correct thread (obviously) and correct cone angle.
For protecting them, I'd suggest a clear aerosol lacquer available from car accessory shops, but try it on an old one first just to make sure it doesn't "argue" with the silver paint.