Leaf lard (pork fat) is the highest grade lard available and comes from around the kidney area of the hog. Lard is used for flavoring fried foods and some pastries. You can purchase leaf lard in the meat department of well stocked grocery stores as well as from your local butcher shop. Typically lard is packed in plastic tubs or 1 pound boxes. In the U.S. Armour is a common brand name producer of lard.
Leaf lard being a "richer" fat gives biscuits and pastries a flakier texture. Some but not all lard products are hydrogenated so check the ingredient list before you buy if this is a concern to you.
If you'd like to see some excellent images of where leaf lard comes from and how you can render it at home check out Grassfood.me - Leaf Lard