Answer 1

Rare candies can be found at these locations:

As visible items: Route 214, Route 218, Route 224, Route 225, Route 230
Wayward Cave, Solaceon Ruins, Mt. Coronet, Victory Road, Stark Mountain

Using the dowsing machine: Route 207, Route 210 (1st Part), Great Marsh, Route 212, Snowpoint City, Galactic Veilstone Building,
Victory Road (1st Part,) Victory Road (2nd Part), Route 224, Route 225
Route 226, Route 228

Chances are this is not a complete list but there is still quite a lot of places to find some.
Source: http://serebii.net/platinum/items.shtml

Comments

Hope I helped!

---

whats wrong with my answer?
its even got the exact same link!

---

artist answerd first so i give him an upvote and it looks exactly similar to which he had written

Answer 2

As visable items:
Route 214
Route 218
Route 224
Route 225
Route 230
Wayward Cave
Solaceon Ruins
Mt. Coronet
Victory Road
Stark Mountain

Using the dowsing machine:
Route 207
Route 210 (1st Part)
Great Marsh
Route 212
Snowpoint City
Galactic Veilstone Building
Victory Road (1st Part)
Victory Road (2nd Part)
Route 224
Route 225
Route 226
Route 228

source:
http://serebii.net/platinum/items.shtml
and personal experience