Look for smooth (with no little "hairs") stiff leaves with deep or protruding veins.
I found these leaves on a bush in Brookside Gardens. Wish I knew the name of the plant...
What to do with your leaf-textured polymer clay sheet? They can be used in many ways to make beautiful jewelry and home decor items. A few ideas to get you started are described in this tutorial of mine:
Polymer Clay Pendants Tutorial Leaf Imprints and Silver Frames
In this tutorial, I describe how to make and use both positive and negative texture plates using natural leaves, how to showcase their texture, and how to make pendants framed in hand-made bezels. By the way, making the bezels, in my humble opinion, is the most valuable part of this tutorial: I am showing my extremely simple, no-soldering, method of making custom-shaped bezels, texturing them, and adding patina.