Ask HN: What are sources to find customers for product interviews?

I have an idea that I am interested in pursuing but want to do it in as lean a way as possible. So, I am looking for small to mid-size businesses to speak with. I have tapped out my resources with companies that I suspect would be interested in the product and would like to find a few more.<p>What resources do you use when trying to find possible customers to interview about a new product?