A really thorough search for grey literature will include a general search of the Web via search engines to ensure you haven't missed anything important. Google is a popular search engine, but you should try others such as Yahoo and Bing to be comprehensive. What appears at the top of search results don't always overlap a great deal between the different search engines.
Consider restricting your search to the .org and/or .gov domains. You can also limit your search to pdf documents (the format many of these reports are stored in).
Let's say you are looking for documents on mobile telemedicine initiatives in Botswana. Your search strategy may look like either of these:
You may want to try both versions in the Google search box below (note that you'll see more about the type of documents and organizations that are returned if you follow the link to Google.com at the end of the search results.)
Searching on the Web means that you will want to take extra care when evaluating these resources.