The best I can think of off hand is to look at the mobile apps that are designed to interact with traditional forums, because they will have directories of all the ones that are integrated with them. For example, Tapatalk and Fora Communities. You should be able to find thousands of forums categorized in those apps? I’ve never used these apps myself, but have heard of them.

Discourse has some of the forums that use it on its website

This is what Google used to be for.