For those who can't find some of these, you can try harvesting or preparing them at home.

Peppers can be hung on a string, ideally in a sunny, dry place, and they'll eventually dry out. From there you can grind them, either manually with a mortar and pestle, or in a coffee grinder (I keep a separate one just for spices). They'll keep better if stored whole. This goes for herbs too, which can even be dried flat on a cookie sheet if spread out (I lut that cookie sheet on top of a radiator at home for a week in the cold season to give it gentle heat that won't cook it).

I saw Taragon mentioned in another comment. If you don't see this commonly, check the fresh herbs section at your loval grocer around Thanksgiving time. Buy extra when it's available and dry it out.

Sumac is harvestable and common in much of the northern US. Poison sumac looks quite different from the edible kind, and should not be easily confused, but google it so you know you're getting the right stuff.

If you're lucky enough to live in a city with an immigrant population, you can find a lot of less common spices at ethnic grocers. Some of the larger ones will even stock spices outside of their particular community's cuisine.

Finally, if you've got money to spend, Penzey's Spices online generally has good quality but it will cost more than other places (some spices benefit more from high quality than others)