I spent a lot of time overlaying provider coverage maps with Zillow listings. Sometimes listings will call out fiber internet or the name of a known provider.

Google queries like rural fiber coop <state> should send you on a good path. There are a surprising amount. In our specific case it was https://www.co-mo.net/ in Morgan County, Missouri. It’s even more niche because the county has no buildings restrictions, so experimental/cheaper building methods are possible.

Email in my profile for more info.