How to make tree leaves suitable for origami?
Try a sewing store.  Ask for lightweight fusible interfacing.  This is a lightweight nonwoven textile generally used to,provide body between two layers of fabric (like in a shirt collar).  Iron it onto a leaf and see what happens. Buy enough to experiment and make some mistakes.

I've fused silk onto the back of patterned origami paper with good results, but the leaf will be tricky as it dries out.