Comment by michaelt
5 years ago
This. You can use 'whois' to find the registrars used by big sites.
The registrar for google.com? microsoft.com? netflix.com? ubuntu.com? reddit.com? amazon.com? They all use Markmonitor.
Of course, markmonitor aren't perfect: https://news.ycombinator.com/item?id=28351432
Other options include 'CSC corporate domains', the registrar for apple.com and twitter.com - and some companies, like facebook and cloudflare, are their own registrars.
One problem startups will run into with MarkMonitor and CSC is that they have a substantial minimum initial purchase. If you've got a domain portfolio, then that works out OK because you're just buying additional years on the domains you've already decided to keep, but it makes it impractical to use them for registry-locking one or two domains.
I believe Cloudflare's initial purchase requirement is much lower.