Where can you find ice stone in Pokemon platinum?

There is no physical ice stone that you can use on a Pokemon in this game. However, if you wish to evolve Eevee into Glaceon then you can evolve it at an icy covered rock somewhere around the Snowpoint area. But you do have to level up when you are around this rock in order for Eevee to evolve.

there is also a item called icy rock. it does some special boost for the TM hail.