Mace blades are the membrane that surrounds nutmeg. The dried nutmeg seed is found inside the nutmeg fruit. The web-like structure that surrounds the nutmeg seed breaks apart into slivers and those called blades.
The dried blades can be ground and used as a spice. Mace blades are much less common to find than ground mace but can be purchased at specialty spice shops. See where to buy link. The flavor is more subtle than nutmeg but can be used when nutmeg is called for. You can purchase the Organic Whole Mace 1.0 oz at Amazon.com
If you are looking for Mace blades locally try a very well-stocked grocery or gourmet shop. Otherwise use the link above for an online source.