Doesn't just about any shop have a Phil Wood spoke cutting jig? They can make any length you want. Mine charges 45 cents per spoke, total.
And, in a related coincidence, I just happened to have picked up a 20" wheel yesterday with a 3 speed hub on it. I have not looked at it closely, so I cannot even vouch for its integrity, but it seemed to work when I played with it. PM me if interested.
jim