Many Cajun recipes call for filé powder.  This is basically ground sassafras leaves - see more at Lowfat Lifestyle.   Sassafras roots used to be used to flavor root beer, but the oil they contain, safarole, was found to be toxic and carcinogenic.   Reportedly, the leaves do not contain safarole -  see info by David. Wm. Reed.   This product does have its drawbacks, in addition to which the flavor may be strange for persons unaccustomed to Louisiana cuisine.

Those that prefer to use another seasoning may substitute "Cajun Seasoning", which has a different but tasty flavor.  You can buy it at many groceries outside Louisiana, or make it yourself.

Cajun Seasoning

3 Tbsp black pepper
2 Tbsp garlic powder
1 tsp onion powder
1 tsp nutmeg 
1 Tbsp paprika
2 Tbsp dried parsley, ground or pulverized
4 Tbsp cayenne pepper (or less if you don't like it hot)
1 tsp dried oregano

Mix well by hand; cap tightly and store in a cool place.   Should keep for at least six months.