Grey (or gray) literature consists of literature or documents that are not available through the usual bibliographic sources such as databases or indexes.
Ask yourself: What do I need to know? Who else cares about that, and might be keeping track of that or talking about it?What region am I interested in? What county is involved, what towns or cities? In other words, who would be producing or keeping this information?
Governmental agencies, non-profits, advocacy groups and others don't necessarily publish their works in conventional library databases. So to search for it, you'll need to be thorough and perhaps unconventional with your research. Start with Google and ook for sites whose URLs end in .gov or .org to help identify non-profit and governmental agencies, as well as .edu sites that may point you toward the unpublished (grey!) work of academic researchers.
However, know that Google can only go so far. It won't bring you back everything, and you may find that you need to search broadly to find sites that you then can examine more closely. You will need to not just search, but browse, which means identifying the organizations and agencies that are relevant to your issue, and then examining their information sources carefully.
Currency: the timeliness of the information
Relevance: the importance of the information for your needs
Authority: the source of the information
Accuracy: the reliability, truthfulness, and correctness of the content
Purpose: the reason the information exists