Nutribullet sells a four-blade replacement blade on its website for $9.96 plus $2.99 for shipping and handling, as of 2015. There are many other sources, including Amazon and eBay.
A new Nutribullet milling blade on eBay can be bought for $7.99 with free standard shipping. This type of blade is designed for grains, herbs and nuts. This item has two blades and is made by the Nutribullet company. A four-blade unit similar to the one sold by the manufacturer is priced at $13.99 with free standard shipping, but this one is manufactured by a third party and described only as being compatible.