you can find them hiding, most of them are invisible so use the app " Dowsing Machine" it might find them. Or, go on you tube its the best.
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There are probly a total of 8 rare candies in Pokemon platinum
Buy an action replay and set it to 999 rare candies
There are only a set amount you can get from the environment without 'randoms' (Pokemon picking them up). For locations, Google it. To have more than the set amount: Have all Pokemon in your party have the ability to pick random stuff up. You can get Rare Candies that way.
Since Pokemon such as Zigzagoon know the Pick Up ability, I would put a whole team of Pokemon with the Pick Up ability at least level 41 (They become just uncommon then instead of very rare) and battle Pokemon or trainers to possibly have your Pokemon hold an item, which may be a Rare Candy.
If you want tonnes of rare candies, I suggest you save up your money to buy gameshark or action replay ds/gameboy advance to hack the game and get extra rare candies.