From Amazon, made by Kartice. I didn't want to appear like I was promoting them, so I didn't post a link right away.

Here you go: https://www.amazon.com/Kartice-ZenWatch-Replacement-Watchband-brown-14mm/dp/B013BVTM3U/

You can find them in other countries' Amazon, too. Just search for "kartice zenwatch 3". I ordered mine from Amazon Germany.