How can I connect to someone who has previously held a specific position at a federal government agency?

I am exploring a tech startup business idea that would provide a data service to the federal government. In order to validate the concept, I need to understand, in some detail, a particular function of a specific government agency. I would like to speak with someone who has previously held a specific role at the agency (ideally in the past couple of years). The role is neither executive management nor entry-level -- somewhere in between. I estimate that there are several hundred people currently with this role and perhaps a thousand people who have previously held the role. The government agency would be a major target customer for my business. Depending on how the agency operates, it could have significant ramifications for how I build my product. It could even render the business model invalid. Before building a prototype and getting in front of the right decision maker, I would like to confirm that the prototype would be worthwhile and solve a real need. Do any of the expert networks like GLG, AlphaSights, etc. specialize in government? I have tried LinkedIn but nobody in my network can introduce me to someone who has held this particular role at this agency. How could I get connected with this person?