Your DNS servers can really be anywhere, I never heard of a rule where you are required to have 2 - I've ran only 1 for years now. This is just my example, I registered with tucows, and https://manage.opensrs.net/
is where I go to set my domain name server. You enter a username and password you received when you register your domain and you get a setup screen to enter the ip address of 1 or more name servers. I'd assume most others work similar. You'll have to call your registrar and get the information if you don't already have it.