Do a Google search for "register domain", or just get a decent web host that doesn't rip you off on domain registration.
Most domain registrars only charge about $9-10 a year to register a .com/.net/.org domain. And even web hosts that aren't registrars usually offer these end-user prices. There's no reason to pay more than $10 per year for a standard domain.
Though it sounds like you just want to take the place of those shady registrars that charge >$10/year for domains.
A domain registration service is about as useful as an AdWords account setup service. Anyone who can read and has access to the internet can register their own domain in less than 5 minutes. It's as easy as creating an e-mail account. It would actually take more time, be more risky, and be more of a pain to go through a third party.
Anyone buying a website for the first time needs to sign up for hosting anyways. And pretty much all web hosts walk you through the process of registering your first domain. Why a separate "domain registration service" is needed is beyond me. That's just one more delay and one more person to hand your credit card information to.
If, say, you sign up with DreamHost for shared hosting, you can just type in a domain name, check availability, and register it with a single mouse click. You won't have an extra account to worry about or have to go in and change DNS records to point to your web host.