Have you search in the retailers for Edison? Find Retailers for Intel® Edison and Intel® Galileo Boards
The Global Ones are = Adafruit, Amazon, Arduino, DF Robot, Makershed, Mouser, NewEgg, Seeed Studio, SparkFun or Tiger Direct. You could check in their websites if they sell the Arduino Expansion Board separately
Please check this thread: Mini-Breakout board on its own?. An user was able to find the Mini Breakout Board alone. I believe you will be able to find the Arduino Expansion Board alone as well.
Thank you for the link.
- the single Arduino breakout board exists under the references ARDUINO.AL.B and ARDUINO2.AL.B (ARK | Intel® Edison Board for Arduino*)
- while the kit including the Edison module has the references EDI1ARDUIN.AL.K and EDI2ARDUIN.AL.K (ARK | Intel® Edison Kit for Arduino*).
Mouser does have both in stock. Just perform a search with the references listed above.
Unfortunately, the ARK | Compare Intel® Products provides no details as the same specifications of the Edison module are mentioned for all the products.
Generally speaking, the lack of clarity when looking for references or performing products comparisons is a potential show-stopper, especially for products aimed at hobbyists.