when you say buy...… do you mean
"where can you register a domain for lowest price"
or
do you mean,
"where can I buy a domain in aftermarket or from another domain seller or expiring domains"
I agree with Biggie, what did you have in mind when you asked your question? Was it related to "registering a domain for the lowest price"? I use https://www.domcomp.com/ for that.