Ask HN: How do you find experts?

When evaluating the merits of the next Big Thing™, sometimes one needs to get an expert opinion, or to find a partner who's deeply entrenched in a specific vertical or skill. Not a co-founder, mind you; a consulting partner or simply a consultant in the purest form of the word.

If I was looking to find someone versed in, say, event planning, I might visit /r/EventPlanners as a first go. Possibly see if there's an SO site where such topics are discussed. Then what? Where else would you start looking? When you've found someone whose brain you'd like to pick, how do you broach that topic?

There are a lot of great ideas floating around here in this community, and I think knowing how to find validation avenues or how to find subject matter experts (especially when the topic is too specific for a gently-marketed MVP) would be a great help for those of us stuck on that classic trap: you don't know what you don't know.

So how do you find people who know?