I did check the Arbor Day Foundation, and while they do have willows, they only show theirs as being hardy to zone 6. It's very confusing; some sites show the (supposedly) same tree as hardy to zone 2.
I'll definitely check with that WA nursery! I stumbled across a nursery in GA that offered some really good prices, but I've never heard of them and somehow, getting a tree from GA to plant in WA seems just a trifle odd, to say the least!