The starting point for your search is your university library catalogue. You should be able to access the electronic resource Dissertation Abstracts International, that includes substantial abstracts of dissertations. Your library can order full copies of the dissertations via interlibrary loan.
Many of the best dissertations are published as books. Oxford University Press and Cambridge University Press both publish series of dissertations by their graduates.
The way to find the best dissertations in a field is to search for those which are still cited in scholarly literature. Alternatively, look for a dissertation written by the leading scholars in your subfield, as their early work probably was good as well.
You can also find copies of every dissertation completed at your university in the library; you can ask your professors what they consider the best ones in your field and read those.