Bought this stone and case, the case works really well to keep the stone hydrated and thus a clean face on it allowing for fast sharpening, rather than getting to the point of rubbing used slurry into the face and then having to redip the stone in water or try to clean the surface if you have a particularly hard stone. It's a standard size 8x2x1 inch.
http://www.toolstation.com/shop/Han.../sd150/Oilstone+Sharpening+Stone+&+Box/p35910
http://www.toolstation.com/shop/Han.../sd150/Oilstone+Sharpening+Stone+&+Box/p35910