If you're in a place that has been attracting visitors for generations, you'll find no shortage of walking tour offers at various prices. But you might also find volunteers who are so enthusiastic about the attraction that they'll take you on a tour for free. While it's true that these tours vary in quality, don't assume that there is always a lack of quality because there is no charge. In fact, sometimes you'll run into knowledgeable guides who simply derive satisfaction from knowing you'll go home with a bit of the same joy they feel for the place.
So ask locally about college, university or civic groups that might provide free walking tour opportunities. Even if it is technically free of charge, it's nice to ask if you can provide a gratuity or contribute to their organization.