Are you asking about how to find more readers for a niche blog? Or are you looking for advertisers who will be willing to place niche-specific ads? I'm not particularly knowledgeable about that latter, but I have a great deal of experience reaching my niche readers. I run and write for a blog specific targeted at mothers of twins, triplets and more. New readers find us in any of the following ways:
* Word of mouth. Whenever I meet a fan, I had them several of our blog business cards. They do our advertising for us!
* Facebook. We have an active Facebook page, with about a 50/50 split between blog content and other related content being shared. Also, friends I make through Mothers of Multiples Facebook groups often end up being devoted blog readers.
* Twitter. A higher proportion of shares from places other than our blog, and more frequent updates than Facebook.
* Pinterest. I try to have an image with each post. Pinterest has become a huge way for us to reach new readers.
* Guest posts. Whenever I publish elsewhere, I make sure to ask for my blog to be linked (if online) or mentioned (if print). Similarly, I invite other bloggers in the same niche to guest post with us.
The most important thing for us is to build a community, so we interact with and get to know each reader as a person, not just a statistic.
I hope that helps!!