Bottle rock can be found in the combini's frozen section although some combini's don't carry it. Don't confuse it with the regular ice as I've done that once.
It doesn't really compare to Gokuri since it's basically a grapefruit drink that's frozen into ice-like shards. It's in white bottle with blue lettering. This drink is perfectly made for natsu and for those of you who like to chew the ice in their drinks when they have nothing better to do. 