Papers from conferences are sometimes collected and published in book form or in academic journals (often in supplements). Often you will be able to locate an abstract of a conference but not the full paper - simply because the full paper has not been published. Contacting the authors or even the organization is one way of tracking down a conference paper.
The term grey literature refers to research that is either unpublished or has been published in non-commercial form. It can include conference papers, theses, government reports, policy statements, working papers, etc. Some limitations of a grey literature search include the possibility of bias, lack of indexing, inability to refine search and often time-cosuming.