Can't say anything about it in western WA, but Ellensburg Pawn usually carries it. you can also order from Buffalo Arms in Ponderay. they carry some very nice, consistent powders like Swiss and German powders as well as Goex
http://www.buffaloarms.com/Content.aspx?PAGE=Black Powder
They'll ship as little as 5 pounds, but there's as usual, lots of restrictions on it. Since i'm closer, if I need powder, I make the drive. It's hard to find though! Black's getting really difficult to find these days, but it's the only thing that'll burn in your flinter!
http://www.buffaloarms.com/Content.aspx?PAGE=Black Powder
They'll ship as little as 5 pounds, but there's as usual, lots of restrictions on it. Since i'm closer, if I need powder, I make the drive. It's hard to find though! Black's getting really difficult to find these days, but it's the only thing that'll burn in your flinter!