What caliber of information do you think an executive board of a $10,000,000 trust will expect in your proposals? One might argue that your job partly depends on the quality of the sources that support the case you are making.
Sources should be of high quality. What does that mean?
Created by experts. These may be people who have terminal degrees, are professional practitioners, or groups who are highly regarded and unbiased.
Verified accurate info. Often why "peer-review" is important.
Well-established, or known for reliability.
Factual, unbiased. Often why studies are "blind" or "double blind."
Time-relevant. Currency is big deal for topics related to country data.
Three places to explore: Web, books in academic libraries, and research databases.