You can buy evolution stones at the Celadon City Department Store on the 4th floor.
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If you mean evolution stones. You can buy them at the shop in Celadon
You can find evolution stones is various places. If you have beaten the Elite Four, go to the Pokeathlon Dome. In the prizes section, there will be evolution stones. Compete in competitions to earn enough points for the one you want.
You need the National poke'dex
The evolution stones in Diamond/Pearl/Platinum are the water stone, fire stone, thunder stone, moon stone, leaf stone, sun stone, dusk stone, shiny stone, and dawn stone. (Sorry if I missed one) And even though this is the opposite answer, the stone to keep Pokemon from evolving (with or without evolution stones) is the everstone. Hope this helps! :)
OVAL STONES CAN EVOLVE- A HAPPINY TO A CHANSEY THUNDERSTONES- Can EVOLVE A PIKACHU TO A RAICHU DUSK STONES-EVOLVE MISDREAVUS TO MISMAGIUS OR MURKROW TO HONCHCROW