My favorite place is to go to where the experts hang out - I usually need a co-founder who is an expert in their subject matter, and I take care of everything else.

Best place I've had success with? reddit.

Mind you, I'm no interloper; I've been part of reddit for over 9 years. I just join the relevant subreddits, see what people are seeing, and slowly but surely I figure out who the even-keeled intelligent people are.

I then reach out and take it from there...