Reports are intended for non-expert audiences who have some knowledge on a given topic. They can also be referred to as "grey literature", which describes materials published outside of the traditional publishing processes used in journalism and academics. You'll want to evaluate their biases and check their credibility as you would any other source.
A key way to locate data is to brainstorm "who else would be interested in my topic?". Often, government agencies, other academics or researchers, and interest groups or non-governmental organizations are good places to locate data.