Novel ways to market for free?

I currently spend $7 a month on upkeep for my kid-safe web browser The Safe Browser, so even just gaining one active user at $9/mo would cover the costs for me, and I think that's a pretty decent goal to have in mind right now. I've always had a terrible time marketing. I ran a semi-successful handmade clothing line through shopify for 2 years, and overall marketing was what I spent almost all of my money on. After being an idiot for too long, I realized I was receiving my sales not through money I spent on marketing, but literally just from making friends with people on instagram.

So now, of course, I'm hesitant to spend any money on online marketing, because I just don't know how it works. I believe I could get into a groove if I found the right audience, but I haven't even tested my app with a small group of maybe 10 people yet.

Should I add people through facebook and practically cold-call them? Should I literally cold-call people using a phone book (older people with kids/grandkids are probably my main audience so people who still hold landlines might be a valuable resource)? Even those simple ideas just sound insane, like I'm some kind of snake oil salesman.

Hit me with as many interesting ideas as possible, even if they might not work! It would be really appreciated! Thanks!