I am working on a new startup and I have to validate it by getting the feedback of my potential customers, aspiring entrepreneurs.
Have you thought about social media (i.e. twitter?) I rely upon twitter for feedback on a regular basis. Another option is to create a targeted list of the 500 people/organizations you specifically want feedback from and send out via email, though as we all know the response rate on something like that is quite small. There are groups on LinkedIn, Facebook, Twitter etc as well that you could tap into.