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Pick your top 10-100 large enterprises, especially those with a major internet presence. Use whois to find the registry they use. See if you can get sales to call you back.

My experience is many years old now, but when Network Solutions's incompetence resulted in our glue records getting changed by hackers, the startup I was at switched to MarkMonitor. At the time, MarkMonitor charged a large premium per domain year, and a significant annual cost to setup 'registry lock' (which prevents the customer or the registrar from making changes to the domain without going through a proccess with the registry), and they had IIRC a $10k/year minimum spending commitment. A lot has changed since then, it's probably worth getting them on the phone if you're serious about your domain, but it's probably still going to be expensive. Get on the phone with at least CSC Domains as well.