What we have is published on https://securitybaseline.eu/datasets openly. Some governments publish lists, and they will be incomplete. In the article we point to our most successful approach: sifting through the (partial) zone file with domain owner information. That delivered thousands of sites the Dutch government didn't even know about.

Perhaps a freedom of information request might also work, but that will take a lot of time to write correctly and does not scale across all governments.