Rare Candy can be foundĀ 
-Invisible (use the Dowsing Machine Poketch App) or in the open at Routes 214, 218, 224*, 225*, and 230*, Wayward Cave, Solaceon Town (outside Solaceon Ruins on the ledge"), Mt. Coronet, Victory Road, and Stark Mountain*
-Use Thief/Covet/Trick/Switcheroo on the Blissey at the end of the Exact Turn Challenge at Backlot Mansion (Route 212 (North))
-Bought in the Battle Frontier* for 48 BP (each)
-Use the Pickup Ability^ of a Level 21 (or higher) Pokemon (Level 21-30 is Rare, Level 31-40 is Very Uncommon, Level 41-100 is Uncommon)

* National Dex Areas
" Catch all 26 Letter Unown then use the Ruin Maniac's Tunnel from Route 214 to reach the area with ! and ? Unown (4 other items inside), the Rare Candy is right outside
^ Meowth, Aipom/Ambipom, Teddiursa, Phanpy, Zigzagoon/Linoone, Pachirisu, and Munchlax can have Pickup Ability (Only Phanpy is guaranteed to have it, Only Aipom/Ambipom, Pachirisu, and Munchlax can be found in the Sinnoh Dex)

You can buy them at the battle tower.

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