"There is no such thing as banana juice. Banana flavor comes from an oil that does not mix well with water. Normally it is imbued in the fatty flesh of the banana. You can buy banana acetate which has a banana-like flavor." - first Google result
It does exist and it's delicious. It's not common in the United States, but for the south east Asian countries I've visited it's commonly available along with other fresh juices.
