Njal.la is the best where you can buy domains. It isn't cheap, but their Whois Privacy are the one of the best. They don't give your information easily.
I recommend don't use .com, and .net if you want do use these TLDs use .org is better than .com & .net domains.
Njal.la
Todaynic
peername.com (get a look into this!!!)
opennic.org
EasyDNS (Only purcharse .org from here) the domain from piratebay is registrated on EasyDNS, you can find if you google it "piratebay domain easydns" they don't will shutdown until a valid court order from Canada.
Piratebay stills online with .org domain. Decent registar that protect you from DMCA complaints. If they receive court order from Canada they will takedown. It only works with .org, forget .com and .net!!
bpw.sc
Domains you should look: .to, .cr, .ru, .su, .cr, .is, .co, .sc and .org (.org is not very safe but is better than using .com or .net)
Opennic and was been used by sci-hub, peername is very interesting, you can give a worth trying, better than nothing. Is more secure and better, but if you still want buying from normal market Njal.la is the best choice at this moment
Unless your content is very aggressive or highly forbidden you should go with Chinese Domain Registars like Todaynic