Afgnic (Afghanistan Network Information Center) provides third and second level domain names and can be registered at www.nic.af. The following 3rd and 2nd level domain names are available at pretty reasonable prices.
Third level domain names (.com.af) are available for Afs (Afghanis) 1000 or $20 and Second level domain name (.af) for Afs 2500 or $50 per year. You can check pricing details at www.nic.af/pricing.htm. One way to register these domains would be through Netlinks.af who provide the same services at the same rates. Before you make your registration request, you can check if the domain is available at whois.nic.af.
If you wanted to pay online using your credit card and have the domain registered under your own name then you can do that via https://secure.coccaregistry.net/.