Aside from those listing above... theyalso have them at Crate and Barrel...
BUT you need to be picky... Most I've seen, even at high end stores have been full of pits! yes, the rock should be somewhat pourous, but nothing bigger than a pin hole or else you are asking for trouble and bacteria!!
I ended up with one I got at Gigante, it's not pretty, but it's the one with the smoothest bowl that they had...