I just did online chat with OfficialDrDrone. They don't have any Spark propellers in stock at this time.
Best Buy has them in stock in retail stores. Website says $11.99 but I got mine for $9.99 (2 props) just yesterday at store. They will also ship free to the store for pick up if store is sold out. Amazon Prime members get free 2-day shipping.
Need to be careful when ordering to know how many you are actually getting: 2 sets? 2 pairs? 2/4 props?