The earliest and easiest place to find Celestial Dew is for purchase from Pidia, Carian Servant located inside of Caria Manor. It costs 5000 Runes. In order to reach Pidia, you must travel to the Three Sisters outdoor area behind the manor (where Renna's, Ranni's, Suluvis's Rise towers are located). Drop from the cliff (see image) and you'll be able to enter a secret room.