Don't overthink it. Look up faculty and try to find one that teaches introductory courses. Send them an email stating something along the lines that you're a non student looking to learn a little more than high school introductory terms. Ask if there's a lecture you could audit or a time like office hours where you could ask questions. A bunch of professors would probably be willing to talk to a flat earther if they were approached on a polite and courteous manner.

If your interest can't be satisfied with a question session, you could look into whether a local university has an option for non-degree students to enroll in classes. That's an option that's frequently not advertised but is pretty common (at least in the US.)