The Biblioteca Nazionale Centrale di Firenze is the central national library in Italy with more than five million holdings of which one million are searchable. Dissertations produced after the mid-eighties are searchable on the OPAC.
If you want to do a search that is more complicated, select the option ricerza avanzata from the menu, type in your search words, select the appropriate operators (and, or, not), and select tesi dottorato from among the options in Filtri applicabili listed in the box Tipo materiale.
Having identified the dissertation you need, try to find it at the Foreign Doctoral Dissertations database of the Center of Research Libraries. If you do not find the title you want, please contact the person dealing with publication requests at CRL. They will buy those dissertations that are not already held in their collection. You also need to fill out an Interlibrary Loan request form in order to receive the dissertation at the circulation desk of a UCLA library.