21mm watch straps are somewhat rare and harder to find than other strap sizes. Thus I’ve found that the selection of styles and materials in this size is not as great as in some of the other more typical watch band sizes. But they can be found, and several examples available online for ordering are shown below. These first items below are women’s band length, to fit 21mm case lugs on the watch.
This next 21 mm watch strap is in a length designed for men.
Was 21 printed on the current strap you have and/or was 21 arrived at by measuring between the watch lugs? If the appropriate style isn’t available in 21 mm online it may be possible to get another close size of watch strap such as a 22 mm width and either try to squeeze it in to fit between the lugs or trim it to make it fit (assuming the watch band is not made of metal) or to get a 20 mm and tolerate a somewhat unsightly gap. Though those approaches are definitely not recommended. For a watch with 21mm between the lugs the best option is generally to get the properly sized 21mm watch strap to fit the watch.
Hope this page helped you in finding a 21 mm watch strap. People who are looking for a 21mm watch band for sale might also find this page useful and informative.