Avocado leaves (hojas de aguacate) are harvested from the native Mexican avocado Peresea drymifolia. The leaves are used both fresh as well as dried and impart a slightly anisey flavor. Avocado leaves are used in soups, stews, tamales and make a great addition to black beans. They are also used for making a herbal tea. (Read more about avocado leaves including misinformation about toxicity.)
What to use if you don't have avocado leaves. There is no perfect substitute but can use one of these options:
Avocado leaves can be purchase in some Mexican grocery stores. There are a few sources for them online as well including Amazon.com.