The best replacement for prune juice is prunes.

The best alternatives to prune juice are rated as following:
★★★★★  prunes
★★★★★  plum juice
★★★  milk of magnesia
★★  miralax
★★  metamucil
★★  dried prunes
★★  laxatives
★  karo syrup
★  grape juice
★  pear juice
★  castor oil
  plum smart
  prune nectar
  cranberry juice
  eating prunes
  stewed prunes

Your alternative for prune juice is not listed? Suggest!
Speaking about prune juice, one probably shouldn't forget about milk of magnesia.

Other search terms related to prune juice and their ratings are:

★★★★★ milk of magnesia
★★★★ miralax
You have in mind a companion for prune juice and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of prune juice in all years. Click on graph for more.
prune juice
plum juice