I asked that question and was told by my Apple store representative that both Nike and Apple have an agreement that they will not sell just the bands. His suggestion was to watch eBay after they go on sale for people selling them.
That's what I'd expect. The bands and a couple of Nike-only watch faces are the only things denoting that version. And the bands are easily recognizable at a distance where the Watch faces wouldn't be.
In the case of Herm?s, they make the bands so selling them separately gets them money (and maybe a slice of the pie for Apple?).