You need to realized that you aren't the first person to market to a specific niche audience.

So go to where all the communities where those users are already handing out! That could be guest blogs, Twitter lists, LinkedIn Groups, Facebook groups, webinars on specific subjects, Quora, Slideshare, Wikipedia, Hacker News, Meetup(s), Craigslist, Skillshare, email newsletters like StartupDigest and events. These are all channels where a large amount of high-quality potential users for your product are every single day.