A product of the Myristica fragrans tree -mace is the exterior web-like membrane that surrounds the nutmeg. Mace is normally sold in dried, ground form and used in many cuisines including Indian, Asian and Europe. It is also sold whole and it is referred to as mace blades.
Ground mace can typically be found in the spices section of most well-stocked grocery stores. You can also purchase it online at Amazon.com. The Spice Hunter Mace.