You can get it everyday. You may not have noticed that there are 2 pages of things you can buy, and you were probably looking at the first page only. It costs 3000 points.
Hope I helped!
You can purchase a Dawn Stone at the Pokeathlon with points on certain days.
you can always earn these too in the pokeathlon dome at specific days of the week
On various days of the week you can buy the thunder stone using your points at the pokeathlon dome which next to national park. North of goldenrod city.
After you complete the National Dex, you can get Shiny Stones on: Mondays, Wednesdays, Thursdays, Saturdays, and Sundays.
If you keep talking to Bill's grandfather on route 25 and show him the Pokemon he asks for, eventually he will ask to see a "Pokemon with 6 tails", meaning Vulpix. Show him a vulpix and he will give you a fire stone. Or, once you get all 16 badges, you can purchase it with your athlete points at the Pokeathlon dome on certain days.
Some days at the pokeathlon
You can buy it in the Pokeathlon on certain days of the week
you can get them at the pokeathlon dome on certain days of the week using athlete points.
You can purchase a Dawn Stone at the Pokeathlon with points on certain days.
You can buy one with Athlete Points at the Pokeathlon Store on some days, or you can get a Waterstone from Bill's grandfather after you show him a certain Pokemon.
pokeathlon on certain days.
as a prize in the pokeathlon dome on certain days
After two days competing in the pokeathlon in the poke dome it was available to me for 1000 points
In Pokeathalon, you need to get 3,000 points. But it will be available in some days.
you can earn it in the pokeathlon dome on specific days
you can always earn these too in the pokeathlon dome at specific days of the week
You can buy them from the pokeathlon, north of Goldenrod city. Just go to the main building of the pokeathlon, then to the right, the shop will sell them on certain days for 3000pp