You can often fit a 22mm strap on there (or a 20mm, but it may show a tiny gap, which is a worse offense).
There are the odd size straps around, you just might need to hunt a bit. Here's one site with a selection:
https://strapsco.com/product-category/sizes/21mm-watch-straps/page/2/?filter_size=21mm

 

Absolutely doable, 22mm straps on 21mm lugs. I had the Certina DS-1 action diver with 21mm lugs for a while and fitted a number of 22mm straps to it. They all fit fine, just had to squish them a bit harder to get the spring bar tool in there to take them off. Here's some pics:

 

Getting a 1mm larger strap off can be a pain if it's a thick strap, but otherwise you're fine. Avoid undersized straps -- if the springbar is a tight fit, the strap can depress it when sliding from side to side. However, I think you're underestimating how many 21mm straps are out there. Sure, it's much less than 20 or 22, but much less than zillions of straps is still a lot of straps.

 

I like wide leather straps so 22mm leather squeezed onto 20mm lugs often.

 

Pending on the strap material, I usually lean towards using a 22mm instead of a 20mm strap opinion.

 

22mm or 21mm straps always look odd on 21mm. IMHO.

 

I use 22's on my 21's.

 

Always better to go larger than shorter. In fact as you see from the pictures above, in many cases they fit better. If you use a softer calf or buffalo, they mold in perfectly.

Most importantly, you have a much larger variety to choose from going with the more popular 22mm size. Everyone who owns a Tissot PRC200 (non- chrono) goes with 20mm instead of the actual 19mm it's supposed to take.

 

Watchgecko do a nice 21mm at a reasonable price.

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