Source: Harald Bøhn, Flickr (edited). All rights reserved
If possible, find the name of the university or other institution where the dissertation was written. This will make finding a copy much easier.
The resources below are listed roughly in order of usefulness. Fuller details of many of them are on the previous tab.
Summon
If you know the dissertation title, a simple search should find it. The content in ProQuest Dissertations & Theses Global: Humanities and Social Sciences and EthOS (for UK doctoral theses) should all be included in Summon, but a separate search to double-check might be worthwhile.
Global ETD
Try this next - Global ETD searches widely online for free copies of dissertations. Also try EBSCO Open Dissertations.
University repositories
Next, check if the relevant university or college has an online repository. You can search for a repository in OpenDOAR; if one is not listed there, look on the website of the home institution, including its library webpages. The repositories guide provides links to search engines for searching across many repositories at once, as well as general information about this kind of resource.
Geographically-based collections
Finding a physical copy
If it is feasible for you to visit the university or college in question, check if its library has a hard copy of the dissertation or thesis. You will also need to check access arrangements. For many London institutions, you will find details on our other libraries guide.
There will occasionally be a hard copy at a library other than the author's home institution. Searching Library Hub Discover (for libraries in Britain, plus Trinity College Dublin) or WorldCat (for libraries worldwide) should find these. Again, you will need to check access arrangements for the library in question.
Contacting the author
Finally, you could look for contact details of the author online and approach them for a PDF. Authors may have many valid reasons for not wanting to send copies to people they do not know, so do not be offended if your request is declined or goes unanswered. Also expect the author to ask you to confirm in writing what you will do - and agree not to do - with any copy they are willing to provide.