Due to its immense popularity the Space Gray finish was in limited quantities at the initial Apple Watch release. For just $33 you can make one yourself using your Silver Aluminum Sport model. This option gives the versitility of owning 2 Apple Watches for a fraction of the cost.
You will need:
Black Leather Loop Band for 42mm Apple Watch – $25
Black Protective Case for 42mm Apple Watch – $8
Black Leather Loop Band for 38mm Apple Watch – $25
Black Protective Case for 38mm Apple Watch – $8