You can get to it by going left of the national park gate
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You can purchase a Dawn Stone at the Pokeathlon with points on certain days.
I'm pretty sure that moonstones only become available in the pokeathlon shop after you get the national dex...
when you go through the route past goldenrod city you will find national park. The door stright ahead goes to the park the door to the left of the gate goes to the pokeathlon dome =]
you can buy one at the pokeathlon dome for 2500 points
On Pokemon Soul Silver and Pokemon Heart Gold, you have to defeat the Goldenrod City Gym Leader, Whitney, and then she tells you that the construction for the Pokeathlon Dome is complete.