Gray literature is written material that is not published commercially or is not generally accessible. Company reports, industry reports, patents, white papers, pre-publications and zines are types of gray literature.
So, how does one find stuff that's not intended for publication or wide distribution? If it's not cataloged in a library or in a database, where do we go? Google.
Search for "$MyIndustry white papers" and you'll get corporate documents,industry reports, etc.
Other good search terms are $MyIndustry +
You can also go do Google Advanced Search(http://www.google.com/advanced_search), and run this same search, but limit the results to PDF, and you'll get the acutal white papers.