There are a lot of registrars out there and you can use whomever you prefer who offers services for the domain(s) you want. For personal/SOHO use, look for established providers who have an interface you like. I use Namecheap. For enterprise, look for an enterprise level providers who have strict modification/access controls (so that someone can't call in pretending to be you to gain access to your account). We use CSC Global.
When you register the domain(s), you'll be asked where the DNS is hosted and this is where you indicate your Infoblox servers. It's best to have at least two 'external' servers which are geographically separate and not on the same network segment.
Most (if not all) registrars require the domain to answer before the registration will be completed and it's a good idea to test resolution anyway. New zones require a service restart, so plan on that if you have defined change windows.
You can test resolution against the server before the registration is complete using dig:
dig <domain.extension> @<NS IP>
1951 
0
