I would recommend creating a new slide with the side-by-side text box
layout, set to numbering bullets and a small font size. Add all source
references to text boxes, expand to 2nd slide if needed.
On the actual slide with the reference add a number after the reference,
select number and superscript. Optionally you could select it and turn it
into a hyperlink that takes you to the appropriate end notes slide for a bit
of interactivity.