1. Go to your local Triathalon shop (lotssa 650cs there)
2. 15mm vs 9/16 is simply the nut size; most axles are (iirc) either 9mm or 10mm. I really find it hard to believe a fork would accept a 9mm but not a 10mm. If that were the case, you could replace the axle/cones in the new wheel.