There are two components to a zafu: the stuffing and the cloth that the stuffing goes in.
Typical stuffings for zafus are buckwheat hulls and kapok fiber. I opted for buckwheat hulls, which I purchased from BuckWheatHull.com
, because they were cheaper and seemed less likely to make a huge mess. 10lbs of buckwheat hulls cost me $35.05 with shipping.
I was going to stop by a local fabric store to find something to use but before I made it to the store my SO picked up a giant piece of fabric that was on the "free rack" in our university's theatre.
You will also need a sewing machine and the ability to hand stitch.