Check if your local university has an agricultural extension and give them a call. They often have resources for the public.

Local nurseries who focus on natives usually carry seeds as well.

Prairie Moon

I just discovered this one, so can’t speak to reliability. Good luck!

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.

Native seed mixes are gonna vary wildly depending on where you live; the same seed mix for say Vermont is not gonna be the same as what you'll get for Oregon. I had luck just searching for "native plant nursery" on Google maps; I live in a place where buying local/focusing on native gardens is baked into the culture so maybe thats why it was that easy for me but the results were for places who exclusively cultivate plants native to my area. Could also be worth checking out a farmers market if you have one near you, there are usually booths that sell Veggie starts and may also sell native flowers and shrubs.