You can get it 3 ways.
1)On the route underneath Goldenrod city. Head down to the huge grass patch South-west of the day-care centre. There will be a trainer. She'll rather give you a water stone instantly, or give you it via poke gear.
2) Get it on the right day for 2500 points at the Pokeathalon store to the far right of the reception desk.
3) Show Bill's grandpa a Staryu. He lives on Cerulean Cape to the very top of Kanto.
Hope this helped.