Here are some of the most frequently used sources for finding public opinion data. Don't hesitate to reach out to a librarian if you're having trouble finding opinion data - it's not always the easiest thing to locate.
One great way to find interest groups is to read newspaper articles about your topic. (Tip: use Proquest US Newsstream database - linked on the first page of this guide!) More than likely, someone representing a particular interest group will be quoted. Pay attention to those organizations and then find them online. You can also try a basic google search to see if there are interest groups on your topic. Also try the resources below: