Your registrar (the place you buy your domains) is where you update your nameservers. If Cloudflare have locked you out then you won’t be able to change them. Other standard registrars will have far less cause, legitimate or not, to lock or disable your account, since they don’t host/proxy your content.
That’s why I said I don’t trust them to not fuck up, not that it’s something that should ordinarily be expected. Additionally, especially considering how the rule of law in their jurisdiction is going recently I wouldn’t assume it will always be this way.