3g Overdrive Pulley

Anyone know where I can get a overdrive pulley for the 3g alternator? My pulley now is about 2.5 inches(about 63.5mm) I'd like to try one about 2 inches( 50-51mm) to increase the rpm on the alternator because at idle the voltage is around 12.8 and lights and everything else goes dim in cold rain with everything running.
Yes batt's are good...

Key in if you have any thoughts or info on this pulley I seek

Thanks

~ub~