The Microsoft Band made quite a splash with its brisk unveiling, and since then stock of all three sizes on the Microsoft Store has completely dried up. At $199 it isn't exactly in "impulse buy" range for many people, and you have to think Microsoft had quite a few of them in anticipation of the launch, so a complete sell out is interesting to see.
If you weren't able to get an order in online your best shot will be to hit up a Microsoft Store in person, though that may not be a guaranteed proposition when you see the pictures of considerable lines that have formed.
Hopefully we'll see online stock replenished in short order.
Source: Microsoft Store