Take a day trip & stop by the Cabelas in Marysville, they have a decent sellectio of powders and other stuff, then up to Skagit Arms in Burlington, They also have a decent selection of powders and other supplies. Then a little farther up north to Kessilrings in Bow. When I need several things I will hit all three of these places.
You can get their info online and call them to see if they have what you want and still check out the other places to see if they have something you might need.