In my personal opinion there are many of domain registrar that provide good quality of domain name registration service with cheap price. I think you have to find cheap domain name registrar through online option. I also want to suggest to use Namecheap domain registrar that provide cheap price to register domain name.
cheap doesn't always mean reliable. So, I'd recommend you to choose time-tested and trusted Domain Registrar that has good reputation. Moreover, try to find any fresh specials or discounts before choosing this or that Registrar.