Trade it from others
How to lots of rare candy's without action replay in pokemon platinum?
You can do this in a double battle: use one Pokemon to hold a rare candy and one Pokemon to move swipe and use it on the Pokemon holding the rare candy.
Keep on battling in battle zone and get Bp then buy rare candies with BP
You go to wifi settings then turn off auto obtain dns. After that go to primary dns and enter this code: 72.232.192.97 (notice you have to have a working wifi signal to do this) after that go in game and then go to GTS and go to where you trade when you go in it will give you a cyndaquil holding a rare candy go back in and out till you have the desired amount of rare candies after your done go back to wifi settings and turn auto obtain DNS back on. Hope this helps it worked for me!