I live in the US, btw.
Check if your local university has an agricultural extension and give them a call. They often have resources for the public.
I just discovered this one, so can’t speak to reliability. Good luck!
Local nurseries who focus on natives usually carry seeds as well.
County ag extension if you’re in a more rural area.
Local nurseries, local botanical gardens, seed exchanges if they exist wherever you are, research your native flora and purchase them from a reputable seed company if nothing else?
Everyone has some banger answers. For sure, your local DNR or extension program should be able to help. Depending on where you live, some Podcasts and their communities (Poor Prole’s Almanac springs to mind) may also have some obscure natives.