I bought the boox page a few weeks ago on a competitor site and found myself with the same problem, no cover and out of stock on the official onyx site.
As an alternative I found a case on Etsy. Artistically made and not too expensive. The finish is perfect. And the delivery was very fast, less than 1 week...
Hope this helps

https://www.etsy.com/ch...

I bought the cover in a shop in the Netherlands.

https://www.e-readerweb.nl/en...