There is no single solution to finding individual evaluation reports or evaluation syntheses. They can be found in the scholarly literature, in databases of systematic reviews, on the websites of organizations, and in general on the web. The resources below will help you get started and the Clearinghouses and Evidence Reviews for Programs of Social benefit directory will lead to more resources. However, one thing to keep in mind when searching for evaluation resources reports, is that keywords like these are useful (and in databases look for evaluation report subject headings or literature types filters):
Instead of diving right into the article literature, try seeing if there's an overview report available.