The case is mentioned on the main product page of the Nova Air C on the international Boox store. Scroll down a ways and you should see it "Magnetic Protective Case".
Here's the direct link to the case, it's just the default Nova Air case that will work for everything in the Nova Air series:
International Store Magnetic Case. That ships from Hong Kong normally.
Update: Boox Europe has it if you're in Europe:
here
Updste 2 OK found it in US Boox store. It's not a very useable site!
here
If you can't get it from any of the Boox stores see what Amazon has got. It looks like they've got the the Nova Air Series case in silver in stock.