Hex Sorry no sets for sale today! But Chessex sells factory second Battlemats and Megamats with 1" hexes, which could be cut to a diamond shape. These work with standard Go stones.
For Amazons I have a relatively simple solution: use draughts for the board (10X10). For the amazons (piece), you can use either: the queens from 4 identical chess sets (idealy), half the pawns from a single chess set or (the least ideal IMHO) the pieces from your draughts set. For the arrows, you can use: go pieces (idealy) or draughts pieces (if not already used for the amazons and not as good as before since you might need up to 3 different draughts sets, although 2 will do in most cases I believe).
Blue Highway Games at 2203 Queen Anne Ave N is dedicated to board games and is not very far from downtown. Gary's Games & Hobbies at 8539 Greenwood Ave N is a ways from downtown but might be another place to look. Then there are lots toy shops around downtown and Capitol Hill which might not have as much of a selection but might still be worth a look. Also I don't know how much you are into Go but on Thursdays the Seattle Go Center at 700 NE 45th St has lessons and is arguably the best Go club in the western hemisphere.