Oran berries can be found in many dungeons or can be bought for 50 Poké. It can be sold for 25 Poké.
Sitrus Berries do not exist in Mystery Dungeon. Actually, Bulbapedia says that they have an ingame effect, but no location is provided. Just assume that they are a lot rarer than Oran Berries.
So, for Oran berries, just keep searching and you'll find enough eventually. Sitrus berries do not exist.