Where can you buy Pokemon contest accessories in Pokemon diamond?

You can't buy them. However, you can exchange your berries for accessories at the shop (Above Pokémart) in Floaroma Town. You can also take either your Pikachu, Clefairy, Jigglypuff, Psyduck, Torchic, Shroomish, Skitty, Pachirisu, Drifloon, Buneary, or Happiny for a walk in Amity Square in Hearthome City and every 200 steps you walk if you turn around and talk to your Pokémon (Press A) it will be holding an accessory. There are also different people around Sinnoh who will give you different accessories. For example there is a woman outside the Great Marsh in Pastoria City with an umbrella that will give you a Piplup Mask.