Congrats on getting the 21. Barsto.com has them in stock but they're pricey but better quality. Lonewolfdist.com you can be put on a list to be notified when available but much better priced. If you want a cheaper route but now I'd check out gunbr0ker, they are a site that allows private owners and small shops to sell their products so they usually have rare and hard to find firearms and accessories, so can probably find one reasonably. Hope this helps and I'm rooting for you to find one know you've been trying hard to get to shoot 40 super, good luck. Oh and don't forget to get a heavier recoil spring, probably 22-24 pounds.

Gunbrоker.cоm

Shaun, Barsto has this here : https://barsto.com/product/40-super-glock-21/

One in stock here
Its spendy
https://barsto.com/product/40-super-glock-21/
Or
Lone wolf has one available for backorder,
lonewolfdist.com/lwd-2140s-blem