There are documents that are not available through the regular (publisher's) channels and that often do not have an ISBN or ISSN number. These documents (such as policy documents, reports, theses and dissertations, conference papers, pamphlets, etc.) are referred to as grey literature.
Searching for these types of resources is not easy. Grey literature can be found everywhere on the internet (at the government websites, at the webpages of organizations and companies, in databases, etc.).
Tips:
Many organizations place the reports they publish on their own website. These publications are often freely accessible and can be found via major web search engines such as Google or Bing.
You can use the following tips when searching for reports:
File type filter: Search with terms that describe your subject and set the filter for file type: climate change filetype: PDF (via the 'advanced search' screen). This decimates the number of results and limits it to the more important sources.
Site filter: If you know which organization publishes important reports for you, you can also set a filter 'site:' where you enter the domain name of the website, for example: climate change site:unep.org (for the United Nations environmental program).
Report keywords: If you still find too many results, you can add that you are looking for reports. For example, add to your search: report OR investigation.
Some publications appear in the form of preprints or working papers (versions of scientific publications that have not yet been officially released). This type of publication is often available earlier and usually free of charge to everyone. Please note: the preprint/working paper may differ from the final (officially published) version.
You can find this type of publications by clicking 'All versions' in a search result in Google Scholar. There are also databases that offer preprints or working papers (see e.g. Dimensions (limit on publication type>preprints), SSRN (for social sciences, in a very broad sense), various preprint servers, etc., etc.)
Conference proceedings are collections of papers (or the summaries of these papers) presented at a conference. They often present preliminary research results and (as a collection) and offer a good overview of the state of affairs in a particular research area (and at that time).
It is difficult to trace (articles in) conference proceedings. Sometimes they appear in book form or as a special issue of a journal. You can find them in search engines and databases such as Google Scholar or Scopus.
If you are looking for an article in a collection of conference proceedings: