For the size and price you won't find a better abrasive stone than these natural, quarried stones from Arkansas. It will sharpen and polish the edge at the same time.
You'll probably find them on the bench of any ski shop in America or in the pocket of most racers and ski tuners.
They are available with a leather case or without. (77mm x 25mm x 5.5mm/3" x 1" x 7/32")