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.