Interesting, I had much the same question. I havent paid a lot of attention to the whole watch thing until now. I’m pretty sure they advertised the first watch as having thousands of combinations so I assumed you could mix and match. I like the navy sport strap but not the rose watch it comes with…
There are a few aftermarket adaptors out there. Get the spring ones rather than screw as they are easy to put straps in and off. I already have collected a few. Quality varies hugely and price is not a determinant.
Here is some photo’s of mu current band.
And the mounting bit which is a couple of screws holding the pins in place for the band. While I haven’t pulled it apart yet I assume that I could simple reuse this for any off the shelf band in the future.
The only downside to this cheapie band is the finish. Clearly pretty cheap with the finish already starting to scratch off where it rubs under my wrist. Next time around I think I would get a stainless steel watch in silver rather than the black as it’s far less limiting in which bands work well on it.
In the meantime I’m looking at something like this which I think works well with the black and give’s it a little colour at the same time 
Source: https://www.amazon.com/Genuine-Italian-Leather-Perforated-Stitching/dp/B0084ODBDM
Or this:
http://www.watch-band-center.com/watchstrap-p19680h254s365-Watch-strap-Trondhei.html