Would you feel comfortable revealing at least the general time period and place? That would be helpful for putting you in the right direction without having to say anything specific.

If there's a university library near you, you could try that out. Some have departmental research librarians who can help you find relevant material.

Also, while intelligent/skeptical friends are good, someone who is an expert in the field is much better. Yes, there are some shady academics out there who might maybe steal material, but that is overwhelmingly not my experience - especially not with historians. It's not like we're going to cure cancer and stand to make bank.

Try r/askacademia.

If you can, try to hunt down academic literature on this subject. Both Google Scholar and any local library will be helpful. Even better would be a university library. If I knew the specific field, I could probably point you towards some academic journals you may find helpful.

With publishers, it's very simple to bring the topic up: send in a manuscript. You say you're a writer, so you may have an agent. Arrange for the submission of a manuscript, and then you've your mark on it.