Sankey diagrams can be tricky to make in PowerPoint, the only route to go is DIY. This slide template contains all the elements you need to create your own. I added rounded elements that you can group to create your own arrows of different sizes.
The file contains 2 slides, one is a finished product (a recreation of this Sankey chart on WikiPedia) where all the colours are set to their correct value, the second slide has alternating colours in it, so you can use how the individual components have been used.
This layout is available as either a PowerPoint or Keynote template.