Go where they are.
Fish where the fish are. Google AdWords and Facebook ads are excellent resources that will help you target online advertising by geography, age range, gender, keywords or areas of interest. They are easy to use, and you can start with a minimal budget, to see how it works in targeting supporters. If you haven’t already, try setting up accounts on these sites, and begin the process of placing an ad. Don’t worry! You won’t have to actually place the ad if you don’t want to, but as part of the process, you will be able to see exactly how many Google searches are done on a certain keyword or how many women between the ages of 25 and 50 are living within 50 miles of Boston — and have indicated an interest in "diabetes" on Facebook (there are 3,360).