What if you need a particular thesis and it’s NOT freely available online? You may use interlibrary loan to request a copy
Request a copy via ILL (Interlibrary Loan) Note: In many cases we can acquire an electronic version, but some theses are often physically owned by only one library at the degree-granting institution. Some libraries will loan their copy, some will not. It’s always OK to try, but it’s not a sure thing that you’ll get it.
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And Last But Not Least It's always worth trying a search engine--Google, Bing, or your favorite--using quotation marks around the title and perhaps the author's last name, if the title isn't unique--to see if the author has posted the dissertation on the web somewhere. It's surprising how often this is the case.
And for cases where you can't find a free copy, but you CAN find contact info for the author, try to get in touch with the author and ask if they'd be willing to share a copy of their thesis with you. Many people are thrilled that someone is interested in their blood-sweat-and-tears project, and would happily oblige.