.com is booked by verisign which keeps account of all .com domain registration. I want to start something like .B9K

So that people can book domain something like www.bitcoin.b9k, www.google.b9k, www.sex.b9k?

I tried to search internet but do not know what keyword should I use to start my search.

Can you tell me how can I book that .b9k extension?
And how much would it cost me?