Not all websites are created equal! Major nongovernmental organizations and various scholarly associations can be rich and authoritative websites, especially for the kind of grey literature not published in mainstream scholarly journals (including white papers, evaluation reports, training materials, and the like). Below are a few useful places to start.
Gray (or grey) literature is literature produced by individuals or organizations outside of commercial and/or academic publishers. This type of non-formally published substantive information (often not formally peer-reviewed; especially important in all kinds of sciences) can include information such:
The sources you select will be informed by your research question and field of study, but should likely include, at a minimum, theses and dissertations.
Most of gray literature is considered less prestigious, reliable, and "official" than publication in a peer-reviewed journal. But they are still fully legitimate avenues of publication. Often they are used to publicize early findings, before a study is entirely complete. Or, in the case of theses, they are published as a condition of receiving an advanced degree. Government technical reports are issued either by agencies that do scientific research themselves or else by a lab that has received government funding. Increasingly, such labs may be required to publish technical reports as a condition of receiving such funding. Gray literature may be cited like any other paper although with the caveat mentioned before that it is considered less "official" and reliable than peer-reviewed scientific papers.
When doing evidence synthesiis, it's important because the intent is to synthesize all available evidence that is applicable to your research question. There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect. Meanwhile, many studies and trials that show no effect end up going unpublished. But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making. While not peer-reviewed, gray literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence.