First of all, the social networks themselves are a good place to search for influencers. You may use in-build searching features, such as hashtags to find the most popular posts and their owners.
The choice of a social network depends on the industry you are working at. For example, if it is fashion, beauty, traveling, sport or lifestyle, your social network #1 may be Instagram. If your filed is software development or marketing, pay more attention to Facebook and Pinterest.