Initially I advertised among friends and former co-workers that I knew needed this kind of service for their own infrastructure. They became my first customers pretty much right away, but my first organic users came couple months later. I marketed primarily by writing articles on LinkedIn.

For the second part of the question - I'd say do some original research and publish it. Hang out where other security people are. Still, the easiest way is to build street cred and professional network while you work for someone else.