You can get Berry Juice two ways.
Give a Wild Shuckle an Oran Berry to hold and wait a while.
Wild Shuckles always hold Berry Juice in HG/SS.
They are not available in Ruby/Sapphire/Emerald/Platinum without a Shuckle.
Also, another good way to get Rare Candies (the best way in my opinion) is to use a Pokemon with Pickup.