This might not be the most popular answer, but I love going with social data in a situation like this. Tools like BuzzSumo, FollowerWonk and Affinio are all ripe with data that can tell you who is active in the space associated with different keywords, topics, etc.
That is a general answer. To give you something more specific I ask, what is the exact data you are looking for? Do you know how you are going to apply it?
The social data can tell you who these people are, what other topics they like, what content is popular, etc. I love that as a starting point, but would love to know more about your goals and objectives.
Research all the social networks, locate the people that are interested in this field.
Its not about just finding the data but providing them with the content information they need. A community needs active members and this can be achieved by providing constant relevant content
Social networks have the ability to hyper target people based on keywords, locations, and hashtags. One of the best networks to drive traffic to your site would be twitter. You can see this video I did on 3 tools in particular that is allowing us to get 50 registrations to our own online community: https://www.youtube.com/watch?v=jBGypRKBUD8
Additionally, LinkedIn is really good for their search features. You will be able to search based on your criteria, and connect with people. The, through private message campaigns, you will send out messages to these connections introducing them into your concept.
I also recently found out how to send direct messages to members of LinkedIn Groups, and this could be another source of registrations to your site.
Hope this helps!
There are multiple organizations where you can farm these names, some paid some free services.
These are all sites where you will find all 3 of your prospective clients pretty active. Or you can start with brokerage sites and farm their online profiles
Marcus & Millichap
Investors I would advise you join sites like AngelList, Investr etc which have open platforms.
LinkedIn and Facebook ( Groups) along with Twitter to a certain extent is a good resource