This guy made his own. Sounds simple and good, a bit labor intensive, but good.
Locate a Sassafras tree and take some branches containing the younger, more tender leaves.
Hang the branches outside to dry (preferably out of the direct sun) for about one week.
When the leaves are completely dry, remove the leaves from the stems and pulverize very well in a mortar and pestle, or an electric coffee grinder as I did.
Pass the powder through a very fine sieve, or a metal coned coffee filter as I did. It was a painstaking process as the mesh was too fine, but it worked to remove all of the little twigs, and tough pieces.
Store in an airtight container and keep out of the sunlight.
Living in Louisiana myself, I wish we had some Sass. trees around us so i could make my own. The process is pretty straight forward as you have seen above. I have had homemade file and I can tell you it beats the store bought stuff hands down. Thats right Zatarains, I'm talking to you! LOL