You can request dissertations held by other libraries through Interlibrary Loan. Many dissertations are available online, so check these resources before you place your request.
Proquest Dissertations and Theses provides free access to American dissertations published after 1997, and contains abstracts for many dissertations published since 1980.
EthOS (Electronic Theses Online Service) provides digital access to dissertations from UK universities. Oxford and Cambridge universities do not participate in this service. Free login and registration is required to search the site.