Sorry, I misunderstood your post and thought that you were specifically looking for NATO straps. I have a number of watches that take 21mm straps and have always been able to purchase them from various sellers, including Amazon. If you want a specific recommendation for a particular color, feel free to send me a message.
I know I should not be wearing this on a freezing cold and damp late may (WTF Mother Nature? Keep this up and I will loose my resistance to the wife’s call to move to Florida!) ....it was calling to me however: