Very few ISPs publish service area maps, never mind accurate coverage maps. The only public data about broadband availability is based on FCC 477 forms, which is notoriously inaccurate. It's so bad that the FCC recently had to walk back claims of broadband coverage for 63 million households. The FCC data is so terrible that you can only use it to rule out coverage areas.

To get any kind of reliable data you'd have to manually try to collect information scattered all over the web. Go over ISP websites, the list of muni broadband networks, carrier portals,...

Even so, ISPs can and will readily deny service, even if they at first say that there is coverage. The only way to make sure is to order service and see if they deliver.

If you have the budget for it, you could consider using commercial services like Fiberlocator, but they mainly cater for the commercial and wholesale markets. I don't know if they cover FTTH. They may, if the network in question also offers business service. They have an API, but that's probably not included in the yearly subscription fee of just under four grand.