Seems that there's not a pure prune juice for bb. You may try apple prune juice from Heinz.
About the poo poo problem, actually I've the same problem as yours some time before 'cos my son doesn't like drinking water ('cos he used to have breastfeed milk in the 1st year) and now he still doesn't like water even after having solid food.
If you encounter great difficulty, I suggest you go visit the doctor and get some a bottle of medicine (I don't remember the name, something like "乳果糖" - correct me if I'm wrong), milky color and it helps to clear the poo poo completely. My son cannot poo poo for 3 days and after taking it, he can subsequently get poo poo (lots of poo poo!) in one go.
After this, I've tried to change his habit by adding more water in the milk (split 3 times of milk into 4) and each time add more water inside and he seems to have better poo poo now (though still cannot have once per day)