The Spice House, with several locations in the Midwest, has the ground sassafras leaves you so urgently require.

For the uninitiated, the Choctaw Indians are said to have been the first group to latch onto the wacky idea of grinding sassafras leaves and using them in cooking. Filé powder has since become an essential ingredient in much Creole cooking, added not only for its flavor (somewhat akin to root beer) but also for its thickening qualities.