if you buy a domain for a website and create a sub domain as GhosT said then you must set it to redirect as your serverip make sure your host can support nameservers some dont let you redirect your sub domain to another site :[
i believe there free ones out there that has there site name in it :[
but its free!
most domains get expensive
if your only using it for the Nameserver then i sugest getting a free 1 because why would you pay $15-30 a year for a nameserver without having a website?
[Edited on 7/9/2007 by ben]