Q. Grey literature?

The Oxford English Dictionary defines grey literature as "documentary material which is not commercially published or publicly available, such as technical reports or internal business documents".

In economics and public policy, the most common type of grey literature tends to be working papers.

For the sciences, check the Scientific Technical Reports at Princeton University guide.

Google and Google Scholar are also good tools for finding many items.

Princeton also acquires grey literature from many think tanks and nonprofits. Specific items can be found by using the Catalog.