According to the Cambridge English dictionary, a dissertation/theses is a major piece of work based in one subject which is usually done to achieve a degree in an academic setting. Dissertations may encompass research that has not been formally published yet, down below is some resources to search, that could be useful in order to find dissertations that are relevant to your subject. They are usually not found until later on in formally published literature and then they are usually in shorter more manageable forms leaving out some crucial details that can be found in dissertations.
Provides access to theses both PHD and Masters from the British library that is taken from various subject areas and universities.
Cinahl complete allows you to limit results to dissertations.
Searchable databases of thesis from various institutes in Europe.
Searchable database of thesis.