There was a suggestion (by Nike, in a tweet), early on, that these were a one-time limited thing, and wouldn't be back. I think that's kind of a stupid idea, but I'm not Nike. It wasn't clear, at the time, if the person answering questions for Nike actually knew what they were talking about.I have the same question. Many of the colors are already sold out everywhere. My girl likes the Violet Dust/Plum Fog one, but I can't find it anywhere! BestBuy removed it from its site as if it doesn't exist.
The Violet Dust/Plum Fog band is really lovely, BTW. I'm wearing it at the moment. I think it's my favorite of the Nike bands. Works well with Space Gray.
There's a for-sale forum here on MacRumors, as well as a Reddit group called AppleBandMarket that does a brisk trade in secondhand sales of Apple Watch Bands of all sorts. You may be able to find a "new in package" or "immediately decided I didn't care for it" one there.