You should be able to find it at Publix with the other cajun/creole seasonings. It may be called "filet powder" rather than "gumbo filet". They're the same thing are are made from ground sassafrass leaves, used both for thickening and for flavor in some gumbo recipes. Filet is usually added at the very end of the cooking process after the gumbo is removed from the heat. There are several brands and all are fine. The brand you'll probably find at Publix is Zatarain's.
File is optional in most gumbos but arguably adds another layer of flavor. Gumbo may also be thickened with roux and/or with okra. If you don't use file but want to season the gumbo similarly, just add a little thyme and perhaps a small pinch of savory during cooking.
I never add file to my gumbo during cooking but do offer it on the table as a seasoning.
"I’m going to break one of the rules of the trade here. I’m going to tell you some of the secrets of improvisation. Just remember — it’s always a good idea to follow the directions exactly the first time you try a recipe. But from then on, you’re on your own." - James Beard