Regional products like pounded fish from coastal tribes were traded to the mountain tribes for goods like beads, horses, roots, and other things they could not produce in their geographic area. Trade goods would eventually make their way across the mountains via Nez Perce, Flathead, or Shoshone tribes and connect the Plains and coast.
If you're interested and want to read more, the journals of Lewis and Clark go into detail about the various trade inter-relations between Native American tribes in the West. Really good read, highly recommended.