how can I talk to a historian or a person who specializes in a certain aspect in history?

Go to a university.

Discover the name and position of a historian who works in the area you need information on and write them a letter, possibly through their publisher or university.

Well, you can ask reddit's "AskHistorians" sub:
https://www.reddit.com/r/AskHistorians/

In meatspace, librarians and a university will help you.

In the course of your research you will surely have come across the names of people who are experts in your particular field. Perhaps you could start with them.

We cannot answer without knowing what aspect in history,