Iron Island - After the Ironworks building goes missing, it washes up here. Check the middle section (just before the small gap) to find another Rare Candy
Victory Road - Found inside, to the left before heading up the last set of stairs
Pokemon Mansion (Basement 2F) - Found in one of the vases
Galactic Veilstone Building (top) - After defeating Cyrus there on your second visit
Galactic Eterna Building (5F) - Given by Charon/Mars/Saturn/Jupiter before fighting Cyrus
Rare Candies can also be obtained throughout the game for defeating certain trainers: Battle Girl Sarah and the Ace Trainers as part of their rematch requests, Palmer after fighting him the second time on Mt. Coronet, Looker's father once inside Team Galactic HQ with Looker (postgame only), Cyrus after entering Route 217.