Are there alternatives to okra as a thickening agent?
Filé powder (ground dried sassafras leaves) would be style-appropriate.   

Fresh okra is only just now coming into season in the American south.   I'm in Tennessee and most supermarkets carry fresh okra for most of the year (though it often looks quite sickly), and frozen year-round.

If it's something you enjoy, you might look into growing your own, as it tastes excellent when it's fresh picked and handled appropriately.